Add write-root-marker role

This will be used in publication jobs in later changes.

Change-Id: I784f81d9999630a643d79f1cfe5c651c40d437d8

roles/write-root-marker/README.rst

@@ -0,0 +1,7 @@
+Write the root marker for an AFS publishing job
+
+** Role Variables **
+
+.. zuul:rolevar:: root_marker_dir
+
+   The marker file will be placed in this directory.

roles/write-root-marker/tasks/main.yaml

@@ -0,0 +1,10 @@
+- name: Write root marker
+  when: "zuul.branch is defined"
+  copy:
+    dest: "{{ root_marker_dir }}/.root-marker"
+    content: "Project: {{ zuul.project.name }} Branch: {{ zuul.branch }} Build: {{ zuul.build }} Revision: {{ zuul.ref }}"
+- name: Write root marker
+  when: "zuul.tag is defined"
+  copy:
+    dest: "{{ root_marker_dir }}/.root-marker"
+    content: "Project: {{ zuul.project.name }} Tag: {{ zuul.tag }} Build: {{ zuul.build }} Revision: {{ zuul.ref }}"