Add OpsCenter support

This PR adds the OpsCenter support. Various things were added, touched or refactored but in essence:
* A new Dockerfile appeared to build the new OpsCenter image  (see `opscenter/` directory)
* The Cassandra Dockerfile now installs (see `cassandra/` directory):
 * SSH server - allows OpsCenter to remotly install agent on cluster nodes. For the story, I firstly tried to install agent directly on cassandra containers, but startup scripts became too complex and gave inconsistent results in the end (unrecognized agent on OpsCenter). Finally, I relied on OpsCenter remote agent auto-installation, this v2.
 * sysstat - needed by the agent to collect system metrics
 * supervisord - to launch Cassandra and SSH server processes simultaneously in a docker container
* Utility scripts were moved into `scripts/` directory
* Documentation was updated to reflect changes

You will need to re-build and push Cassandra Dockerfile and new OpsCenter Dockerfile.