REVIEW: [NEXUS-6744] disable eviction of bundles when system is shutting down

This ensures that those components registered with the system at the time shutdown begins are kept around at least until shutdown is complete. This should make things more stable while still allowing plugins/features to be added and removed while Nexus is still running.

https://issues.sonatype.org/browse/NEXUS-6744
http://bamboo.s/browse/NX-OSSF207-1 :white_check_mark: