NEXUS-5711: Auto routing periodic update is chatty about unsupported repositories

Reason was that periodic updater (running every hour currently)
did not filter the involved "maven repositories" as it got them
from registry. The result contained shadows too, that are NOT
supported by automatic routing.
Merge pull request #857 from sonatype/nexus-5711-autorouting-unsupported

[2.6] NEXUS-5711: Auto routing periodic update is chatty about unsupported repositories