Add replace_member test

Completely replace the root ensemble: root/2/3 -> 4/5/6
Then replace the new ensemble with the old members: 4/5/6 -> root/2/3.

Also add riak_ensemble_peer:tree_info/1 for debugging and add
replace_members_test to TESTS list.
Merge pull request #45 from basho/test/re-add-members

Add replace_member test

Reviewed-by: andrewjstone