REVIEW: [NXCM-5392] Use a P2 composite specific content class

https://issues.sonatype.org/browse/NXCM-5392
https://bamboo.zion.sonatype.com/browse/NX-OSSF15

The changes will allow P2 composite groups to have as members any P2 proxy, P2 legacy group and P2 composite group, while P2 legacy group will not allow members of type P2 composite group but will allow P2 proxy, P2 legacy group (as it was till now)

The pull includes a bunch of fixes related to content class compatibility. The fixes were required as up till now we never had a content class that was compatible to another content class.