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.  
I wonder if this is the private IP and if yes, how can you reach it?
Are you using nova-network?