NEXUS-6232: Prevent concurrent execution of createrepo on same data

Added exclusive lock around MD generation (invocation of createrepo)
that assures that on same repository instance only one
Generate Yum Metadata task will run, while the others will remain
blocked waiting for the lock.
Merge remote-tracking branch 'origin/master' into NEXUS-6232-concurrent-createrepo
NEXUS-6232: Adjust mocks in UTs

As created mocks lacked some responses.
NEXUS-6232: Adjusting mocks
Merge pull request #334 from sonatype/NEXUS-6232-concurrent-createrepo

NEXUS-6232: Prevent concurrent execution of createrepo on same data