Add support for use_keys and key_file netmiko arguments.

[pixelrebel]

Tested with force10 s4810 switch. Full disclosure: there's an issue with netmiko/paramiko that requires a valid password for the key exchange to work (at least for the s4810 in my testing). In any case, it's nice to have the key option available.

[jedelman8]

What's getpass being used for here in the context for adding use_keys and key_file for netmiko?

[pixelrebel]

@jedelman8: Oops, good catch. Those were temporary debugging lines. I will remove these.

Also, there's an issue with required_together arguments. Either host, username, password or host, username, use_keys, key_file would be required together. I'm not sure how to write that.