Container image config file for builds optional

Make the config option optional for the container image build process.
In previous releases we have shipped a basic yaml file that contains the
list of container to build. We have moved to generating using a jinja
file instead which we ship so we don't need to manage this static file
going forward.

Change-Id: Id689c54f393affe103082e329efaf4ebf376e397
Depends-On: https://review.openstack.org/#/c/639846/

tripleoclient/v1/container_image.py

@@ -127,11 +127,11 @@ class BuildImage(command.Command):
             metavar='<yaml config file>',
             default=[],
             action="append",
-            required=True,
             help=_("YAML config file specifying the images to build. May be "
                    "specified multiple times. Order is preserved, and later "
                    "files will override some options in previous files. "
-                   "Other options will append."),
+                   "Other options will append. If not specified, the default "
+                   "set of containers will be built."),
         )
         parser.add_argument(
             "--kolla-config-file",