Extend ensemble_remove_node2 to test shutdown

ensemble_remove_node2 uses an intercept to prevent a riak_ensemble
related transition that is necessary for nodes to completely exit and
shutdown after removal. In fact, testing for this scenario is the
entire point of this test, since it is testing logic that was added to
solve basho/riak_core#572 and that logic prevents nodes from exiting
until that transition occurs.

However, even without this new logic, there is an unrelated
riak_ensemble related bug that can trigger a race condition that also
prevents nodes from shutting down.

The good news is that other changes made as part of the solution to
solve basho/riak_core#572 also fix this unrelated bug. Therefore this
commit extends ensemble_remove_node2 to remove the intercept at the
end of the test and verify that the removed nodes do actually end up
exiting as expected. Thus, the test now tests for both the negative
and positive scenarios and serves as a test against future regressions
that stall node removal/shutdown.