Added ability for mongo role to do simple replica set clusters
By default this should keep the role the same.  The only catch with this implementation
is that the ansible mongodb_user module doesn't handle slaves particularly well. So if you
have a cluster than any task that creates a user will succeed on the master and fail on all
slaves.
Merge pull request #299 from edx/carsongee/mongo_cluster_support

Added ability for mongo role to do simple replica set clusters