Allow missing rhn.conf file

[cbosdo]

What does this PR change?

Now that rhnLog looks for the apache user and group in rhn.conf unit tests are failing if the file is not present. Use openSUSE values as default if rhn.conf is not available.
Note that this should never happen on a running server instance!

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• No tests: already covered

• DONE

Links

Fixes #
Tracks # add downstream PR, 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

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Re-run a test

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• 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"