[NEXUS-5829] Validate that (if present) repository is registered as yum repo via capability and is a hosted repository

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #32 from sonatype/NEXUS-5829-only-run-mergemetadata-task-on-hosted

TRIVIAL: [NEXUS-5829] Validate that (if present) repository is registered as yum repo via capability and is a hosted repository