upstream ci: Avoid scheduling tests that will not be executed.

Currently, all tests are scheduled to execution, even those that are
not executed due to being absent from the list of enabled tests
configured in the IPA_ENABLED_* variables. The tests that are not
executed are marked 'skipped'.

This patch change this behavior by not scheduling tests that are not
configured to be executed. It means that tests not the IPA_DISABLED_*
lists are not skipped anymore, but not scheduled to be executed. If
any test is in IPA_ENABLED_* lists, only those tests are marked for
execution. A side effect is that there is no visual feedback on which
tests were not executed, as disabled tests are not evaluated anymore.

Also, when IPA_SERVER_HOST was not set, all tests were skipped, but
an error should raised in this case, as there are no hosts to run the
tests against.

This patch modifies this behavior to fail the test with an exception if
IPA_SERVER_HOST is not set.