Filter null/None service names

Filter null/None service names

To enable use of the service_names lists in overcloud.yaml we need
to strip any null/None values or list processing becomes cumbersome.

These happen because we're currently disabling some services via
OS::Heat::None in the resource_registry, it may be possible to remove
when we've got a fully composable approach to generating the
*Services lists.

Change-Id: I8ef53b4279d93850c207c73aab0d75095a2497a2
Partially-Implements: blueprint custom-roles