Host availability check isn't good enough

[ewxrjk]

The current way to test whether a host is available is to see if SSHing to it succeeds. If it's not available then no backup is attempted, and this is silent.

However a host may be up but misconfigured in some way that means that SSH fails. In that case the host would not be backed up and the operator would not know there was a problem.

Other possible approaches:

• ping it
• connect to some port

This should be configurable.