fix for RoutingWithProxyRepositoryIT

As it is about Central, but central prefix
is processed during boot, and the outcome is left
to chance of concurrency (at my machine, it was 1:3
of executions)
Merge pull request #891 from sonatype/RoutingWithProxyRepositoryIT-fix

fix for RoutingWithProxyRepositoryIT
Merge remote-tracking branch 'origin/master' into NEXUS-7292-scheduler-rework