Support SSH commands in 'ec3 ssh'

[gmolto]

When using ec3 ssh cluster-name to log in to a cluster it would be convenient to support passing parameters in order to remotely invoke commands via SSH (just as the ssh command supports).

Expected functionality:

As an example, issuing

ec3 ssh cluster-name ls -l /tmp

would result in listing the contents of the /tmp dir in the front-end node of the cluster identified by cluster-name

[micafer]

Solved in the master branch.