MB-1143: refresh pool details on server group updates

I.e. so that ?v trick works

Change-Id: I46f6b9feb8f212aafc1238d1899a2f841bb10fd6
Reviewed-on: http://review.couchbase.org/29869
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-1143: added internal unittest for server groups API

Change-Id: I83d70fd86ceb75c1a5dbc277b64edce9530bd134
Reviewed-on: http://review.couchbase.org/29870
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-1143: added api doc for server groups REST API

Change-Id: Iabaaa4383568df4146808e312074f49873acab1e
Reviewed-on: http://review.couchbase.org/29873
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>