REVIEW [NX-487] Support customisation of Jetty in Karaf (plus related bootstrap work)

Consolidate bootstrapping code, remove nexus-webapp, get Jetty conf working in Karaf
Some cleanup still required, but the build is no longer based around building WARs first.

http://bamboo.s/browse/NX-OSSF127-2 :white_check_mark: 
Sure, I can remove them