Add parameter ExtraHostFileEntries

This allows passing hosts entries from an external stack, or
that references other external nodes.  This is useful in the
case where you want to split e.g controlplane nodes from compute
or similar.

As an example you could do something like this to generate an
environment file:

openstack stack output show controlplane HostsEntry | grep controller\
 | sed "s/|//g" | sed "s/^ */    - /" \
 | sed "1s/^/parameter_defaults:\n  ExtraHostFileEntries:\n/"

Change-Id: If26f87e3384e242ae637650adce7c5dba1611cdf

overcloud.j2.yaml

@@ -59,6 +59,10 @@ parameters:
       The DNS name of this cloud's provisioning network endpoint. E.g.
       'ci-overcloud.ctlplane.tripleo.org'.
     type: string
+  ExtraHostFileEntries:
+    default: []
+    description: List of extra hosts entries to be appended to /etc/hosts
+    type: comma_delimited_list
   ExtraConfig:
     default: {}
     description: |
@@ -617,6 +621,7 @@ resources:
             - ""
             - {get_attr: [{{role.name}}, hosts_entry]}
 {% endfor %}
+        - {get_param: ExtraHostFileEntries}
 
   allNodesConfig:
     type: OS::TripleO::AllNodes::SoftwareConfig