NEXUS-7371: support nexus-features property

https://issues.sonatype.org/browse/NEXUS-7371
http://bamboo.s/browse/NX-OSSF396-1 :white_check_mark:

If the `nexus-features` property is set then only the features named in that comma-separated list will be installed. the `nexus-features` property is not set then Nexus will attempt to discover available features using a heuristic to ignore different editions (if they happen to be also installed) and the boot feature.

Expectation is that the `nexus-features` property will only be configured in the final distribution when we want to enable a subset of the available features (for example when selecting features in the installation wizard)
k, though think we eventually want to get away from this... so we aren't bitten by adding a feature that doesn't have the right artifactId and having to re-id things just to make them show up.