docker: Add metadata_settings to optional parameters for yaml validate
docker: Add metadata_settings to optional parameters for yaml validate

This section will be needed for TLS-everywhere. So it should be added as
optional in the yaml-validate.

Change-Id: Ic6ea563b6c8e454cb51f640bb5aaa3adda82a5dd

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates