Zuul v3: move local.conf writing to a role

This is likely to become more complex in the future, go ahead and
compartmentalize it in a role.

Change-Id: I14bd0da1243ed3cf5ab5d639a71aa9b11ec7bb11

playbooks/pre.yaml

@@ -1,6 +1,3 @@
 - hosts: all
-  tasks:
-    - name: Write job-specific localrc variables
-      copy:
-        dest: /tmp/dg-local.conf
-        content: '{{ devstack_local_conf_contents }}'
+  roles:
+    - write-devstack-local-conf

roles/write-devstack-local-conf/README.rst

@@ -0,0 +1,7 @@
+Write the local.conf file for use by devstack
+
+**Role Variables**
+
+.. zuul:rolevar:: devstack_local_conf_contents
+
+   A string that should be written to devstack's local.conf.

roles/write-devstack-local-conf/tasks/main.yaml

@@ -0,0 +1,4 @@
+- name: Write job-specific localrc variables
+  copy:
+    dest: /tmp/dg-local.conf
+    content: '{{ devstack_local_conf_contents }}'