Add support for OpsCenter

[pofallon]

Would you be open to including support for the community edition of OpsCenter? I could see this materializing similar to the following:

docker run --name=n1 -d tobert/cassandra -agent
docker run --name=n2 -d tobert/cassandra -agent -seeds $IP
docker run --name=ops -d -p 8888:8888 tobert/cassandra opscenter -seeds $IP

The first two commands would run the Datastax agent on those two nodes. The third command would run OpsCenter (not Cassandra) and use the OpsCenter REST API on localhost to register the cluster using the value provided to -seeds.

From my own experiments, adding the agent and opscenter to the docker image increases the overall size by about 217Mb. If this is unacceptable I suppose a second tagged image could be created (tobert/cassandra:opscenter) but maintaining two tagged versions would be more of a pain going forward.

If you're open to it, I'll work on a pull request for it.

Thoughts? Thanks!