We no enforce formatting of json schema, this was causing a test
failure:
```

======================================================================
FAIL: test_schema_neoppod_instance_neo_input_schema_json_format (slapos.test.test_json_schema.TestJSONSchemaValidation)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/srv/slapgrid/slappart0/srv/testnode/bpy/inst/test0-0/parts/slapos.cookbook/slapos/test/test_json_schema.py", line 80, in run
    content.splitlines())
AssertionError: Lists differ: ['{', '  "$schema": "http://js... != ['{', '  "$schema": "http://js...

First differing element 87:
'            "description": "Storage type. Defaults to MySQL if available, else SQLite.",'
'              "description": "Storage type. Defaults to MySQL if available, else SQLite.",'

First list contains 3 additional elements.
First extra element 119:
'  },'

Diff is 5422 characters long. Set self.maxDiff to None to see it.

----------------------------------------------------------------------
```

We still want to check the signature of the reference .data file and of the index

/reviewed-on !453

  Present a reduced sub-set of parameters for simpler users.

It will be used on python2.7

from https://docs.python.org/2/library/subprocess.html :
POSIX users (Linux, BSD, etc.) are strongly encouraged to install and
use the much more recent subprocess32 module instead of the version
included with python 2.7. It is a drop in replacement with better
behavior in many situations.

/reviewed-on !476

slapos.core test suite needs `sensors` command in $PATH
slapos.core@7467ef4c (comment 71574)

/reviewed-on !479

Since some software releases have unit tests, it is good idea to run the
tests for each commit on slapos branch.

This SR follow slapos-testing SR in a way how tests are grouped and run.

Fixed erp5.util is used to expose good test suite names.

When debian 9.4 was latest version, we wanted the url to expand to
http://cdimage.debian.org/cdimage/release/current/amd64/iso-cd/debian-9.4.0-amd64-netinst.iso
now we want
http://cdimage.debian.org/cdimage/archive/9.4.0/amd64/iso-cd/debian-9.4.0-amd64-netinst.iso

no reason to use http

apache mirrors only have latest version of tomcat, but
archive.apache.org have all versions.

Asserts Caddy issue https://github.com/mholt/caddy/issues/2394

 * keep the start date
 * don't remove the file before writing in it (use > directly) so that the file always exist
 * format the date with ISO 8601
 * format all the lines the same way

This will let the monitor promise read this file easily

The recent changes in rdiff backup are not backward compatible.

Because we have no choice and test do pass I merge right away.

https://nexedi.erp5.net/test_result_module/20181217-239F0A22/view

/reviewed-on !477

See sqlite-mixin.cfg to skip the building of MariaDB.

Since usage of cryptography in "caddy-frontend/test: Switch to cryptography"
certificates are programmatically  done.

Caddy has its own approach to security and browser support, and it is
trusted, so keep it this way. If incompatibilities would be found, they
will be tackled by each one.

Current way of testing caddy-frontend SR is enough.

It is done in "caddy-frontend: Restart 6tunnel services on SR upgrade"