[KNIFE-422] Add option for specifying identity file for SSL gateway when creating ec2 server
[KNIFE-422] use SSH configuration for SSH gateway polling
[KNIFE-466] extract get_ssh_gateway_for, configure_ssh_gateway, and update ssh_gateway help
Update documentation / logs for KNIFE-464, KNIFE-466
Merge remote-tracking branch 'victorlin/enhancement-ssl-gateway-identity' into adamed/knife-422-466

Conflicts:
	lib/chef/knife/ec2_server_create.rb
KNIFE-422: Make get_ssh_gateway_for more verbose
KNIFE-422: Fixes for Net::SSH options hash and add unit tests

The options hash passed to Net::SSH::Gateway.new should be passed an array of :keys.

http://net-ssh.github.io/ssh/v1/chapter-2.html#s3
Doc updates for KNIFE-422 and KNIFE-466
Fix spelling error in CONTRIBUTIONS
Merge pull request #213 from opscode/adamed/knife-422-466

KNIFE-422, KNIFE-466: Use SSH Config for ssh gateway options