Add cloud-specific functions (separate from actions)

This changes the way that -a/--action is handled internally, and keeps -f/--function functions from being called by -a, and keeps actions from being called by -f.

Sample usage for this style looks like:

    salt-cloud -f ec2 show_image image=ami-fd20ad94

If such a function were to be called with --action, an instance name would be required to make the call, but not used.