REVIEW: NEXUS-6647: Tell the user when NFC entry will expire

The log and describe output will contain _when_ the NFC is about to expire.

Issue
https://issues.sonatype.org/browse/NEXUS-6647

CI
http://bamboo.s/browse/NX-OSSF96
Nothing is computed inside the debug call now, so `isDebugEnabled` check is redundant.