Making Nexus570IndexArchetypeIT stable.

Changes:
* As legacy ITs already had (unsatisfied) expectation about
  "sync" nature of Indexer related operations, added a flag
  to IndexerManagerEventInspector that makes it "blocking" or
  "async" depending on a system property. To be used in ITs.
* removed obsolete "mavenIndexerBlockingCommits" property
  from Jetty8NexusBooter, as code handling this was removed along
  with other notable changes happened in MI 5.x. Simply put,
  this property had no effect since then.
* Using the newly added key in Jetty8NexusBooter, to make
  Index related event inspector become "blocking".
Merge pull request #852 from sonatype/fix-nexus570indexarchetype

[REVIEW] Making Nexus570IndexArchetypeIT stable.