Force update ring in ensemble_util:build_cluster()

Once riak_ensemble_manager:enable() is called, we need to call
riak_core_ring_manager:force_update() so that the members will be
created and added to the ensembles trying to get a quorum. During ticks
in core, new members are created only if the ring has changed. There is
a race that can sometimes prevent the members from starting and thus the
quorum from being achieved indefinitely. This small change to the test
infrastructure resolves this issue, but it still requires fixing in
riak_core and/or riak_kv.