Gatekeeper: documentation

[enmand]

Documentation is required for the Gatekeeper. The Torus documentation should provide how to run the Gatekeeper, and how to bootstrap a machine. Some notes for documentation:

torus gatekeeper start -- start the Gatekeeper instance
torus machines bootstrap -- bootstrap a machine against a Gatekeeper instance

--auth and --url are required to provide the authentication provider type (currently only aws), and the URL of the Gatekeeper instance. --role and --org are required only if the Gatekeeper instance does not have configured defaults for Torus.