Do undercloud container prepare in external_deploy_tasks

This change moves docker and docker-distribution installation from
step 1 to host_prep_tasks, then runs container prepare as
external_deploy_tasks.

Container image prepare needs to run before any steps which require
container images, and it always needs to run on the undercloud because
it only ever populates the undercloud registry. Docker needs to be
installed in host_prep_tasks, since it is just before
external_deploy_tasks step 1.

The aim is to move these prepare tasks into their own service file so
they can be run during undercloud install and overcloud deploy, as an
alternative to having a dedicated mistral action for the overcloud
prepare.

Change-Id: I2c82b6829e574424067130d1a369ff30030fb4bc
Blueprint: container-prepare-workflow

puppet/services/docker-registry.j2.yaml

@@ -60,10 +60,8 @@ outputs:
               - 8787
               - 13787
       step_config: ''
-      host_prep_tasks: []
+      host_prep_tasks:
-      deploy_steps_tasks:
         - name: Install, Configure and Run Docker Distribution
-          when: step|int == 1
           block:
           # NOTE(bogdando): w/a https://github.com/ansible/ansible/issues/42621
           - set_fact:
@@ -77,6 +75,10 @@ outputs:
           - include_role:
               name: container-registry
               tasks_from: docker-distribution
+      external_deploy_tasks:
+        - name: Container image prepare
+          when: step|int == 1
+          block:
           - name: Create temp file for prepare parameter
             tempfile:
               state: file

puppet/services/docker.yaml

@@ -80,10 +80,8 @@ outputs:
       service_name: docker
       config_settings: {}
       step_config: ''
-      host_prep_tasks: []
+      host_prep_tasks:
-      deploy_steps_tasks:
         - name: Install, Configure and Run Docker
-          when: step|int == 1
           block:
           # NOTE(bogdando): w/a https://github.com/ansible/ansible/issues/42621
           - set_fact: &docker_vars