Implement PlenaryCollection.add()
Implement PlenaryCollection.add()

This moves the logic for figuring out the right plenary classes into
PlenaryCollection, allowing further restructuring later.

Change-Id: I462cb9f69825888557cc17f470059f5d3e8df00a
Reviewed-by: Dave Reeve <6a84ee3ae0690d4584b99406fb3076392171f50e@morganstanley.com>

guillaume-philippon/aquilon,quattor/aquilon,guillaume-philippon/aquilon,guillaume-philippon/aquilon,quattor/aquilon,quattor/aquilon