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.

Similar (if not same) solution already introduced on another Yum related task
https://github.com/sonatype/nexus-oss/commit/7994e32dbd0a802eda5a555c232e1187eba35ab5
as part of
https://github.com/sonatype/nexus-oss/pull/201

Issue
https://issues.sonatype.org/browse/NEXUS-6232

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF43