NEXUS-7789: Fixing IT and fixing bug it revealed

After fixing the ignored Nexus4066TaskMutualExclusionIT it
revealed that mutual exclusion is "less strict" that it should,
it does not consider transitive group members.

IT fixed and enabled, and bug fixed.

Issue
https://issues.sonatype.org/browse/NEXUS-7789

CI
http://bamboo.s/browse/NX-OSSF437
Fixed in f5412d5a811c9b3747dfa8bbf0d8f30ceafefc9a