improvement(sdcm/sct_config.py): perform repo checks for proper backends
improvement(sdcm/sct_config.py): perform repo checks for proper backends

Do not verify config options values that are related to package repos
on the backends which do not use these options such as "K8S" and
"siren"-based.
So, it will allow us to avoid false negative results of checks.

scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests