Add the collection of libraries to interface with MAVLink systems mavsdk

This is the case in debian 11 when we don't have python2 installed.

See merge request nexedi/slapos!1247

this package is python3

We no longer need backports.lzma now this test is python3 only

Now that all the problems of running a different version of python in
slapos-node and within the software and now that slapos-node always run
on python3, we no longer need to have two separate test softwares and
two separate suites for python2 and python3 - we have one test suite
for all our softwares, regardless of the python version used by these
softwares and this test suite use python3 to run the tests code and to
run slapos-node

On modern systems it is not installed and anyway slapos should not
depend on host system programs.
Basically same fix as b964b95b (software/fluentd: Fix SR test on Debian
11, 2022-01-04)

we'll only run the test on python3

See merge request !1228

mca--static.cfg provides a statically linked version of mca.
"dep--" means it does not compile all the building dependencies.

Sometimes backends process less than 15% of requests. For this test to
test that the requests are distributed on all backends, testing that
each backend process at least one request should be enough.

Fixed indentation.

See merge request nexedi/slapos!1243

See merge request nexedi/slapos!1245

See merge request nexedi/slapos!1229

our shacache uploading code for pypi cannot handle the prefix 0 in the version number right now.
it will upload the version 2020.4.5.1 instead of 2020.04.05.1

pango needs glib/glib-mkenums, and glib-mkenums needs python2.7:
-mkenums: /usr/bin/env: ‘python2.7’: No such file or directory

Direct logs to instance's var/log and call postfix's logrotate command,
instead of the slapos-hostile syslog.
Unfortunately, rotated logs will stay in that directory, unlike the
de-facto standard for slapos.