Fixed ssh-key auth

Added the ability to use IdentityFile from .ssh/config file
Also re-enabled default of underlying ncclient allow_agent and look_for_keys
By default ncclient will:
	1. Try passed in keyfile
	2. Look for keys loaded in agent
	3. Look for default keys in user home
	4. Try passed in user/pass combo
Re-added proper logic for allow_agent
Functional test for SSH keys (non-agent)
Merge pull request #281 from shermdog/sshident

Fixed ssh-key auth