refactor: improve support for ostree systems

[richm]

The dependency on ansible.utils.update_fact is causing issue with
some users who now must install that collection in order to run
the role, even if they do not care about ostree.

The fix is to stop trying to set ansible_facts.pkg_mgr, and instead
force the use of the ostree package manager with the package: module
use: option. The strategy is - on ostree systems, set the flag
__$ROLENAME_is_ostree if the system is an ostree system. The flag
will either be undefined or false on non-ostree systems.
Then, change every invocation of the package: module like this:

- name: Ensure required packages are present
  package:
    name: "{{ __$ROLENAME_packages }}"
    state: present
    use: "{{ (__$ROLENAME_is_ostree | d(false)) |
      ternary('ansible.posix.rhel_rpm_ostree', omit) }}"

This should ensure that the use: parameter is not used if the system
is non-ostree. The goal is to make the ostree support as unobtrusive
as possible for non-ostree systems.
The user can also set __$ROLENAME_is_ostree: true in the inventory or play
if the user knows that ostree is being used and wants to skip the check.
Or, the user is concerned about the performance hit for ostree detection
on non-ostree systems, and sets __$ROLENAME_is_ostree: false to skip
the check.
The flag __$ROLENAME_is_ostree can also be used in the role or tests to
include or exclude tasks from being run on ostree systems.

This fix also improves error reporting in the get_ostree_data.sh script
when included roles cannot be found.

[richm]

[citest]