obs/generic/: Add notes on the differences between the slapos-node packaging system and the generic packaging system.

build-scripts/:
- configuration_information.sh:
    - Automatically detect the osc user. Deduce the osc project.
    - Improve space handling in the variables name.
- Reorganize slightly and clean. Add " " around variables.
- Do nothing after running buildout. It allows to work on other projects during the compilation time. Create a new stage.
- Rename the scripts. Add and rename some variables.
- Restore the buildout files retrieving using git clones (rather than copying a model in the current filesystem).

templates/compilation-templates/: Change a macro according to a new variable name.

Makefile: Update.
- Supports both numbered and unnumbered names.
- Update for renaming and stage addition.
- Add an after_build target and fix some details.

obs/generic/: Improve MCA compilation. Fix some bugs. Add, clean and update distribution and compilation files or templates.

obs/generic/templates/compilation-templates/mca/: Improve MCA compilation. Copy more files from it.
- Fix and improve the Makefile.
- Copy more things from openssl(-dev), i.e. everything except the links to the certificates (in parts/ca-certificates/).

build-scripts/clean_build_tree.sh: Fix failure when $RUN_BUILDOUT_DIR/go did not exist.
build-scripts/bootstrap_buildout.sh: Fix a bug in the backup mechanism.
templates/:
- Fix compilation-templates/mca/Makefile.in
- Change distribution-templates/debian_defaults/dirs.in to fit the mca case.

distribution-specifics/:
- Change the "Version" field in the .dsc
- fluent-bit: Add distribution-specifics clean the compilation templates.

Add support for dep--mca: MCA with light build dependencies.
- Add distribution-specifics/dep--mca/
- Add templates /compilation-templates/dep--mca/

Add support for dep--mca--static: Statically linked MCA ith light build
dependencies.
- Add distribution-specifics/dep--mca--static/
- Add templates /compilation-templates/dep--mca--static/

Add support for dep--fluent-bit: fluent-bit with light build
dependencies.
- Add distribution-specifics/dep--mca/
- Add templates /compilation-templates/dep--mca/

build-scripts/*bootstrap_buildout.sh: Improve the backup mechanism of the tarball directory taking into account the Go modules handling.

obs/generic/: Support fluentbit-plugin-wendelin as a separate tarball. Fix some bugs and update files.

obs/generic/templates/compilation-templates/mca/Makefile.in: Support fluentbit-plugin-wendelin as a separate tarball.

obs/generic/: Fix some bugs. Update files.

build-scripts/clean_build_tree.sh: Fix failure when $RUN_BUILDOUT_DIR/go did not exist.
build-scripts/bootstrap_buildout.sh: Fix a bug in the backup mechanism.
templates/:
- Fix mca/Makefile.in
- Change debian_defaults/dirs.in to fit the mca case.

obs/generic/build-scripts/clean_build_tree.sh: Add permissions in go/ directory in order to delete it.

Note: Allow to use custom distribution files instead of the template-generated ones.

templates/: Split into compilation templates and distribution files templates.

distribution-specifics/:
- Remove mca, lunzip and fluent-bit files: they should be generated the same way.
- Add examples files.

build-scripts/:
- Add some variables.
- Add a script for the distribution files.

Makefile:
- Add a stage for distribution files.
- Add a build target: no distrib files generation, no obs.

Warning: General targets tested with lunzip. Reversibility with clean_* untested.

build-scripts/:
- Backup bin/buildout and restore it so that it is not "rebootstraped" when sent to OBS.
- Change some variable names, add a sed regex.

This provides a tool to help package a software from buildout using OBS.

See merge request nexedi/slapos.package!161

It has more chances to exist than /usr/bin/python executable

This commit also simplifies the preparation of the cache for OBS compilation.

The slapos-node package can be built in any Linux distribution (old or
new).

WARNING: Do not use PYTHON make variable since it messes up with compilation. Instead
use SLAPOS_BOOTSTRAP_SYSTEM_PYTHON which explains the idea of the variable
and does not overrides make's PYTHON variable in other places.

also do not check for invalid certificate in slapos.cfg anymore. It's
been more than 3 years that the certificate is invalid. All our machines
are updated.

For CentOS_8 & CentOS_8_Stream, I give up:
- nothing provides python, python2, python3 or /usr/bin/python
  (actually, there's a python2 package on CentOS_8_Stream, but only to ship
  a /usr/bin/python2 that tells the user not to use Python 2... wtf ?)
- by default, there exists /usr/libexec/platform-python and it's functional
  but the build of openssl requires any of python{,2,3} in PATH, which means
  that a probable solution is to:
  - conditionally depend on 'python' only `%if 0%{?centos_ver} != 8`
  - change Makefile.in to add a symlink to /usr/libexec/platform-python
    in a temporary directory that we add to $PATH during the build
- ... but it's probably fine to use packages for CentOS_7

- Make it compatible with Python 3.
- Install setuptools in a way that's compatible with slapos.rebootstrap
  (otherwise, rebootstrapping fails because signature dependency to
  setuptools changes).

playbook: imt-vm-bootstrap playbook will be called only until success, then it is never called again

This reverts commit 910d2148.

Actually, we will simply stop executing the playbook totally after it is
well configured.

This reverts commit dd750800.