Add a role to write a root marker

This is a single task within the existing afs prep role, but we
need to be able to do it alone for certain special doc builds,
like infra index page generation.

We could use include_role in the future to de-duplicate this, once
zuul_json supports that.

Change-Id: Ia6c6257cb1479abef79bf919cb6ad25ea6828d90

roles/write-root-marker/README.rst

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+Write the root marker for an AFS publishing job
+
+** Role Variables **
+
+.. zuul:rolevar:: root_marker_dir
+   :default: src/{{ zuul.project.canonical_name }}/doc/build/html
+
+   The documentation build directory.  The marker file will be placed
+   in this directory.

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

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+root_marker_dir: "src/{{ zuul.project.canonical_name }}/doc/build/html"

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

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+- name: Write marker text
+  copy:
+    dest: "{{ root_marker_dir }}/.root-marker"
+    content: "Project: {{ zuul.project.name }} Branch: {{ zuul.branch }} Build: {{ zuul.build }} Revision: {{ zuul.ref }}"