Acceptance tests

Fixes #315.
The `ping` output is quite noisy. Either:
 * dump it to /dev/null
 * find a `ping --quiet` option 
 * ....or probably better still, use the socket module to attempt a TCP connection to the SSH port of the node. After all, that's what you actually require for the tests for proceed.

See https://github.com/ClusterHQ/flocker/blob/0.2.0/flocker/node/testtools.py#L16-L28