Check the value of the --color argument

Make sure it's either `auto' (the default), `always' or `never'. If it
does not match any of the values, raise a Docopt exception that causes
it to print usage and exit.

Tested:
- $ sudo ./bootstrap-vz --color=invalid manifests/gce.manifest.yml
  Value of --color must be one of auto, always or never.
  Usage: bootstrap-vz [options] MANIFEST

Signed-off-by: Filipe Brandenburger <filbranden@google.com>