CBQE-1834: add 'group' option to cluster task

supports adding nodes group using 'group' option in cluster tag.

"0" :
{
    "name" : "rebalance_in",
    "desc" :  "rebalance_in",
    "cluster" : {"add" : "2", "group" : "group1"}}
}

if group does not exist it will be created.

Change-Id: Ief1887709cedf5531a247d0368daac6a3c7820e3
Reviewed-on: http://review.couchbase.org/32998
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Thuan Nguyen <soccon@gmail.com>