[WIP] Add KeyName option for servers

[hironobu-s]

This adds KeyName property to CreateOpts struct, and It will be able to create the instance with ssh public key.

https://developer.rackspace.com/docs/cloud-servers/v2/developer-guide/#post-create-server-servers

[coveralls]

Coverage decreased (-0.008%) to 79.964% when pulling 4786f049d093c84905ed506488023f1c7dee3365 on hironobu-s:add-key-name-option into 3dc52174c26169aeae623cb224278dfac476ca08 on rackspace:master.

[jrperritt]

In OpenStack, keypairs are an extension. Use the keypairs package: https://github.com/rackspace/gophercloud/blob/master/openstack/compute/v2/extensions/keypairs/requests.go#L14

[hironobu-s]

Thank you for your advice!