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

A trivial change, surrounding the meat of for cycle into an IF condition.

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, and auto routing was logging that 
fact, spamming the log at hourly rate.

Fixes
https://issues.sonatype.org/browse/NEXUS-5711