Refactor bootstrap-os (#10983)

* Remove leftover files for Coreos

Coreos was replaced by flatcar in 058438a2 but the file was copied
instead of moved.

* Remove workarounds for resolved ansible issues

* boostrap: Use first_found to include per distro

Using directly ID and VARIANT_ID with first_found allow for less manual
includes.
Distro "families" are simply handled by symlinks.

* boostrap: don't set ansible_python_interpreter

- Allows users to override the chosen python_interpreter with group_vars
  easily (group_vars have lesser precedence than facts)
- Allows us to use vars at the task scope to use a virtual env

Ansible python discovery has improved, so those workarounds should not
be necessary anymore.
Special workaround for Flatcar, due to upstream ansible not willing to
support it.