Add public binding to support DNS-HA.

This binding is required so that the DNS-HA code can find the
address to use for the hostname specified by os-public-hostname.

This also deprecates the os-internal-hostname and os-admin-hostname
options, as there is no binding to use them with.

Change-Id: I57609c5ab641e2ae6377c6fbad5c9e7b8cf6495c
Closes-Bug: #1742548

config.yaml

@@ -136,6 +136,9 @@ options:
     type: string
     default:
     description: |
+      [DEPRECATED] Use os-public-hostname to specify a hostname for
+      openstack-dashboard.
+      .
       The hostname or address of the internal endpoints created for
       openstack-dashboard.
       .
@@ -148,6 +151,9 @@ options:
     type: string
     default:
     description: |
+      [DEPRECATED] Use os-public-hostname to specify a hostname for
+      openstack-dashboard.
+      .
       The hostname or address of the admin endpoints created for
       openstack-dashboard.
       .

metadata.yaml

@@ -7,6 +7,8 @@ description: |
 tags:
   - openstack
   - misc
+extra-bindings:
+  public:  # Only used to allow specifying a hostname for DNS-HA.
 series:
   - xenial
   - bionic