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.

Previously it only worked with Debian 10. This commit reflects the
removal of the apt package 'python-pip' in Debian 11 and the change
of the i2c bus number.

There were still two issues which are solved with this commit:

1. Previously the firewall prevented the installation of Fluentd
   (because no connection via http/https was possible).

2. Previously the sensor couldn't send data to the IoT gateway (
   the opened port wasn't sufficient).

When importing the slapos playbook after running re6stnet-install role,
the user defined computer_name will be ignored and will be overridden
by 'set_facts', line 16 in re6stnet-install/tasks/main.yml. Therefore
the slapos install role is directly called (instead of importing the
playbook) which seems to solve the issue.

(due to activated firewall)

(wrong indentation)

(wasn't possible because of too restrictive ipv6 firewall)

Add restrictive firewall settings to avoid any security risk.

Bug appeared due to wrong pip version. Therefore the version has been
set explicitly.

Now the Python script name is consistent with the file name in the
Fluentd config file as described in the learning track.