overcloud/service_names: merge service_names & enabled_services

When building ServiceNames, make sure we merge the YAQL results of both
enabled_services & service_names queries; so we don't end up with
duplicates.

Change-Id: I1f062b8b9f936e6fbf2febf64244e91b59b8ba1b
Related-Bug: #1855138
(cherry picked from commit f5d8df1)

overcloud.j2.yaml

@@ -643,7 +643,7 @@ resources:
       type: comma_delimited_list
       value:
         yaql:
-          expression: let(root => $) -> $.data.extra_services.items().where($[0] in coalesce($root.data.enabled_services, [])).select($[1]).flatten() + coalesce($root.data.enabled_services, [])
+          expression: let(root => $) -> distinct($.data.extra_services.items().where($[0] in coalesce($root.data.enabled_services, [])).select($[1]).flatten() + coalesce($root.data.enabled_services, []))
           data:
             enabled_services: {get_attr: [{{role.name}}ServiceChainRoleData, value, service_names]}
             extra_services: