Add the undercloud mistral user to the docker group

This is required for mistral actions to perform image prepare
operations (pull, build, push)

Change-Id: I301ea04e4dbb3809ce247c0c620b0f087dedb5f9
Blueprint: container-prepare-workflow

elements/puppet-stack-config/puppet-stack-config.pp

@@ -580,6 +580,10 @@ include ::mistral::keystone::auth
 include ::mistral::db::sync
 include ::mistral::api
 include ::mistral::engine
+ensure_resource('user', 'mistral', {
+  'name'   => 'mistral',
+  'groups' => 'docker',
+})
 include ::mistral::executor
 include ::mistral::cors
 include ::mistral::cron_trigger