Add token secret to intermediate registry

An upcoming change will add JWT authentication to the registry;
prepare for that by establishing a server-side secret for use
in signing the tokens.

Change-Id: Ibaa15dd0c4b0d797f01a1886186fdc021dc990fa

playbooks/roles/registry/templates/registry.yaml.j2

@@ -3,6 +3,7 @@ registry:
   port: 5000
   tls-cert: /certs/domain.crt
   tls-key: /certs/domain.key
+  secret: {{ registry_token_secret }}
   users:
     - name: {{ registry_user }}
       pass: {{ registry_password }}

playbooks/zuul/templates/group_vars/registry.yaml.j2

@@ -5,6 +5,7 @@ registry_swift_tenant: 123456
 registry_swift_region: DFW
 registry_swift_container: intermediate_registry
 registry_swift_secretkey: testsecretkey
+registry_token_secret: test_secret
 registry_password: testpassword
 registry_tls_cert: |
   -----BEGIN CERTIFICATE-----