MB-1143: suggest rebalance if server groups have changed

This commit modifies ns_rebalancer:unbalanced logic to return true if
vbucket map options differ from last successful rebalance. And we now
store vbucket map options hash in order to detect that.

Change-Id: I479c038fffe116805e20b51e2af41ccdbbab30b2
Reviewed-on: http://review.couchbase.org/30224
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>