MB-1143: use server groups for vbucket map generation

Change-Id: I9175dcb1fd5b2954b007f614049726e5af5f1fee
Reviewed-on: http://review.couchbase.org/29953
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>