Adding support for Consul tests

This is probably a very newb PR so please forgive. I've cargo-culted most of this to make it work based on the etcd work. The one thing is it seems Jepsen does value based CAS while Consul does index based CAS. I've worked around to first do a read to get the index and then do the CAS operation. Not sure if there is a way to improve this by passing the index from a read into the CAS operation. Thanks!