Comment 4 for bug 1626955

@skramaja: the use case is when you want to deploy some extra role, but without having to constantly rebase a custom roles file (e.g when the packaged roles_data.yaml changes, such as to add, remove or rename existing OpenStack Services.

Perhaps "merge" is the wrong term - I mean if say you pass roles-extra.yaml, and it contains a "Controller" role and a "ServiceApi" role (where the latter is running the openstack API services, which are no longer running on the Controller role), we should accept the roles-extra.yaml definition of "Controller", overwriting the one from the default roles_data.yaml (but all other non colliding roles in roles_data.yaml should remain).