Add volumes to the cloud module, state, and client

This is to add stuff like volume_create and volume_attach to the cloud module, and then volume_exists and volume_attached to the cloud state.

@techhat

As we discussed on Friday, it still uses the salt/utils/openstack/nova.py and I added some things to that to make it look more like the stuff that comes out of apache-libcloud.

Please let me know what you think.  I believe it is ready to go.

Thanks
Daniel