Cleanup and remove systemd use in the setup scripts

[cbosdo]

What does this PR change?

Remove unused code in the setup scripts in order to ease refactoring them. This commit removes the need for systemd running as PID1 to run the setup script.

This also means that mgr-setup has to be called before running the real container and starting everything.

Note that some of the cleanup removing non-SUSE distributions handling, which shouldn't be a problem as Uyuni is now only shipped as a container built on a single openSUSE distro.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• CI needs to be adjusted

• DONE

Links

Issue(s): #
Port(s): # add downstream PR(s), if any

• DONE

Changelogs

Make sure the changelogs entries you are adding are compliant with https://github.com/uyuni-project/uyuni/wiki/Contributing#changelogs and https://github.com/uyuni-project/uyuni/wiki/Contributing#uyuni-projectuyuni-repository

If you don't need a changelog check, please mark this checkbox:

• No changelog needed

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Re-run a test

If you need to re-run a test, please mark the related checkbox, it will be unchecked automatically once it has re-run:

• Re-run test "changelog_test"
• Re-run test "backend_unittests_pgsql"
• Re-run test "java_pgsql_tests"
• Re-run test "schema_migration_test_pgsql"
• Re-run test "susemanager_unittests"
• Re-run test "javascript_lint"
• Re-run test "spacecmd_unittests"

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