[NEXUS-7768] Replace Kenai modello config with a capability

https://issues.sonatype.org/browse/NEXUS-7768
http://bamboo.s/browse/NX-OSSF436

In addition to using capability for configuration I refactored Kenai impl to be similar to how Crowd is implemented regarding configuration. Some methods do not make sense now but will be used when we will add  an UI 
checkNotNull()? (looks like is/setRealmActive() will NPE)