REVIEW [NX-485] remove plugin repository references

This PR removes the last references to a 'plugin-repository'. Instead we let Karaf manage the bundles and use the extender pattern to assemble the application. OSGi start levels are used to maintain the distinct activation phase after booting NxApplication, but before NexusInitializedEvent (think of them like init.d run-levels when starting OS services).

We take advantage of karaf-maven-plugin:install-kars to pre-seed a separate test feature repository, which is then overlaid onto the base assembly as and when required.

https://issues.sonatype.org/browse/NX-485
http://bamboo.s/browse/NX-OSSF165-2