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