Get SSH host parameters from ~/.ssh/config file

No need to specify the key_filename etc., allows different settings per user. We might not all have named our private key the same.

 - Should the config file be specified per host (as I did it now), or as a global setting, which can be overridden (`client.py -F ~/.ssh/other_config run`)? I picked the first method because it was the easiest, but the second might be better.
 - Who should take precedence: the config file or the host class? Currently I overwrite values from the config file if they are specified in the host class.
 - This reads the config file once for every host, this could probably be optimized (moved somewhere else).