Add support for identity files as valid SSH options
to our remote plugin. The use case for this is connecting
to Amazon EC2 instances using the provided keypair (.pem
files).

avocado run --remote-hostname my-machine.com --remote-username myuser --remote-key-file /path/to/myfile.pem passtest
Signed-off-by: NLucas Meneghel Rodrigues <lookkas@gmail.com>