NEXUS-7371: remove the automatic feature discovery in favour of a strict list of features to install

https://issues.sonatype.org/browse/NEXUS-7371
http://bamboo.s/browse/NX-OSSF405-1 :white_check_mark:

Still considering if it makes sense for the features list to be stored as a property or kept in its own file.
`it-resources` files are filtered, so this will be replaced by the value specified in the pom.xml (where the additional test plugins/features are actually defined and listed). Otherwise someone could just edit the pom.xml and miss this file which is what really controls the embedded Nexus process.

Note that the modern IT setup is better in that it takes the IT config and produces the appropriate configuration for the launched Nexus process. Unfortunately we're limited in the legacy ITs to using maven filtering (unless we spend time writing and maintaining our own maven plugins for the legacy tests.)