REVIEW [NX-160] generate Karaf features for Nexus base install + plugins

http://bamboo.s/browse/NX-OSSF41-1 :white_check_mark: 

Note: this just generates the Karaf features (one for Nexus base and one for each plugin), https://issues.sonatype.org/browse/NX-395 will update the distribution to consume these features.