Undercloud hosts entries in overcloud nodes

With IPv6 the overcloud nodes cannot pull images from the
undercloud using an IPv6 address. By adding hosts entries
for the undercloud a name can be used instead of the IPv6
address.

The parameter 'UndercloudHostsEntries' is populated with
entries from the HEAT_HOSTS section of /etc/hosts on the
undercloud.

Related-Bug: #1836578
Depends-On: Id9271460b20e602f935f1eddd70f989cd605b1f4
Change-Id: Iac6efde9dd283906274d95c3a239b4b882ec052e

overcloud.j2.yaml

@@ -76,6 +76,13 @@ parameters:
     default: []
     description: List of extra hosts entries to be appended to /etc/hosts
     type: comma_delimited_list
+  UndercloudHostsEntries:
+    default: []
+    description: >
+      List of undercloud hosts entries to be appended to /etc/hosts. The
+      value is populated with the HEAT_HOSTS entries on the undercloud by
+      tripleoclient when running deploy.
+    type: comma_delimited_list
   EndpointMapOverride:
     default: {}
     description: Can be used to override the calcluated EndpointMap
@@ -785,6 +792,7 @@ resources:
       value:
         list_join:
         - "\n"
+        - {get_param: UndercloudHostsEntries}
         - - if:
             - add_vips_to_etc_hosts
             - {get_attr: [VipHosts, value]}