MB-1143: added server groups

Because removal of nodes requires removal of node from it's server
group I decided to create new compat mode: [2, 5].

Once all nodes are 2.5+ our node addition code becomes aware of server
groups. Node ejection code is always aware of it.

Change-Id: Ica7e6e0824c7e4b8decf4d3cfc81d64aaf21d2c8
Reviewed-on: http://review.couchbase.org/29867
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-1143: added REST API for server groups

Change-Id: I11eb4a39445baaa15a05bb36390e1bffdb59e2dd
Reviewed-on: http://review.couchbase.org/29868
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>