Add ssh check to check_nova-instance.py

This commit enhances check_nova-instance.py with an *optional* additional check and fixes some proofreading items in the whole script.

* The check attempts to SSH into the created instance using the provided keypair details.
* It assumes SSH can reach the instance on the first IP in `self.instance.networks.itervalues().next()`.
* It has a configurable timeout (45 seconds by default).

Apologies for lack of granularity, I wasn't tracking the file when I made changes. This should really be two commits.  
Does `socket.getfqdn()` always returns the same value as `hostname -f`?  I'm not attached to `hostname -f` and can change this if that's what's best.