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)
Ease of development/testing so developers can quickly try out plugins without having to keep touching this property. This way we can select an initial sub-set of features in the final distros (maybe populated by the installer) while still supporting the old-style discovery of Nx2.