* add strategy mitogen_linear when installed mitogen

* add small docs

Rename playbook file

The raw action executes as a regular Mitogen connection, which requires Python on the target, so add strategy: linear to bootstrap-os role playbook.

* add mitogen to  CI test
fix typo

* enable mitogen test on deploy-part1 tests
change version from master to release
download tar.gz archive

* run all CI tests with mitogen

* disable mitogen with upgrade CI tests

* enable mitogen on CI tests via env vars

* disable mitogen on CI test by default, enable on some different OS

* disable mitogen CI test on centos8
(get error  /usr/bin/python: No such file or directory)

* bump minimal ansible version to 2.8.0

* check ansible version in separate playbook

* Lint everything in the repository with yamllint

* yamllint fixes: syntax fixes only

* yamllint fixes: move comments to play names

* yamllint fixes: indent comments in .gitlab-ci.yml file