Introduce a simple tool to format JSON for software instance definitions.
This tool is inspired by `python -m json.tool`, but has the following differences:
 * keep the order of keys instead of sorting alphabetically (because we sometimes put the most important options at the top)
 * indent with 2 spaces

Run the tool on all the repository and commit the changes, with this command: 
```
for x in $(find . -name '*.json') ; do  echo $x; ./format-json $x $x; done
```

/reviewed-on nexedi/slapos!392

As check-free-disk-space promise is implemented in plugin directory, add it
to list of assertions and drop it from promise list assertions.

Also already existing monitoring plugins are asserted.

Before we move this code to a more suitable place, start by agreeing with small differences in the implementation.

This takes all the nice changes from caddy-frontend/tests/ but not what was specific to caddy or to testing slave instances.

/reviewed-on nexedi/slapos!389

This release should be now stable enough for release

/reviewed-on nexedi/slapos!391

In preparation for moving this to a better place, use the same code in
all the places where this code was duplicated.

Reconcile all implementations, document a little bit more the methods
and the future plans.

It is combination of requestoptional and serialised.

In order to avoid needless inheritance create JSONCodec and use it, also
with renaming Serialised to RequestJSONEncoded.

Also provide a test asserting that important log is seen in case if no expected
return is set.

Add testfixtures module to catch logging.

if saucelabs_dict is present, then generate delegate runTestSuite to launch test with saucelab
else default one is created

  it  clean up....

  Up to some extent we would like to set diskcache=none and diskaio=native

This reverts commit 2c79bee1 because it depends on rdiff-backup version 1.3.4nxd4 which is not stable yet.

This reverts commit 9b3dfa01

On runner importer script, when updating database, set timeout to 5 seconds (same value as the default in python sqlite3 module: https://docs.python.org/2.7/library/sqlite3.html#sqlite3.connect). If database is locked by another process we wait a bit instead of raise immediately "database is locked". Importer script will raise if database is locked for more than 5 seconds.

/reviewed-on nexedi/slapos!384

they are already pinned in stack/slapos.cfg

When slapos.libnetworkcache is not installed, then shacache is not
enabled, so SR will no longer build when there's a dead link.

/reviewed on nexedi/slapos!379

See https://www.erp5.com/group_section/forum/No-test-results-rtSJ3H4fpC

/reviewed-on nexedi/slapos!382

https://github.com/v8/v8/wiki/Building%20with%20Gyp

aspell-en-dictionary, groonga-normalizer-mysql, file-msooxml, apache-antiloris are not shareable, they write files in other parts.

With new rdiff-backup version, it is possible to speed up push process by ignoring UID/GID changes (as it will always be different because of different partitions)

/reviewed-on !378