Child group repository with only one yum enabled member breaks yum metadata merge

Create a set of yum enabled repositories:

GroupA --> GroupB
     |       |
     |       |--> Repo1
     |
     |--> Repo2
     |--> Repo3
     |--> Repo4

Because "GroupB" only has one member mergerepo is never run against it. Consequentially it contains no yum metadata. This breaks running mergerepo against "GroupA", it will fail every time with error:

Could not merge repos: Could not setup merge repo pkgsack: Cannot retrieve repository metadata (repomd.xml) for repository: repo1. Please verify its path and try again

I think the simplest fix for this would be:

1. Implement NEXUS-6801 (prevent generation of sqlite metadata)
2. If a group only contains one member copy that member's metadata and rewrite the URL in the primary.xml.gz file