NEXUS-5658: Copy+Paste of FSDirectory selection from Indexer

This commit merely copy-pastes the FSDirectory selection
from the DefaultIndexerManager of nexus-indexer-lucene-plugin

Squashed back-port of following commits:
https://github.com/sonatype/nexus/commit/f46668f678d66bd73891964cf97dc61f694acbed
https://github.com/sonatype/nexus/commit/1fb1a6b0fbe86e2cc22cd372f2c5cf23da4235db

Original pull against master:
https://github.com/sonatype/nexus/pull/797