Update ensemble tests to work with auto-activation

Prior to this commit, the various riak_ensemble related tests would
manually enable the consensus system on one-and-only-one node in a
given cluster in order to work around issue basho/riak_core#571.

This commit changes the tests to work properly after the above issue
has been fixed.

In addition to removing the call to `riak_ensemble_manager:enable()`
that is now handled automatically by Riak, this commit also removes
a few `wait_until_stable/2` checks against 1-node clusters. These
checks no longer apply, since Riak is now designed to only enable
the consensus system after the cluster contains at least 3 nodes.

Related pull-request: basho/riak_core#601