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
Minor: this code could be moved into a `Set<String> transitiveHull(String repositoryId)` method, and the check to see if there's an overlap can then be changed to:
```
!Collections.disjoint(
    transitiveHull(getConfiguration().getRepositoryId()),
    transitiveHull(taskInfo.getConfiguration().getRepositoryId())
```
which avoids creating an intermediate collection that's only checked to see if it's empty.