cloud does not allow arbitrary ping machines

[Curtis Hovey]

As seen at
    http://reports.vapour.ws/releases/issue/58c94eea749a5632a574cf11

some clouds do not allow arbitrary ping to instance machines. The network-health test needs another strategy to very machines are reachable.

We discussed changing charms to provide a trivial python web server on port 7353 that returns a text or json value. This solution was also discussed as an improvement to the expose to actually exorcise a conversation instead of reading juju status.

The suggested solution does not work for unexposed. This might be a problem. We know that clouds/cloud-init tend to permit ssh, but such a test need to know when the substrate requires --proxy. Maybe we can find a command to show routes to a mchine.