Add role to ensure per-node output dirs exist

To move the output interface onto the nodes, we need to make sure the
dirs exist on the nodes so people can count on them for putting stuff in
them.

Change-Id: I969218f59c3fc237e1b85f016a7d0c1548a0b073

roles/ensure-output-dirs/README.rst

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+Ensure output directories are in place
+
+**Role Variables**
+
+.. zuul:rolevar:: zuul_output_dir
+   :default: {{ ansible_user_dir }}/zuul-output
+
+   Base directory for collecting job output.

roles/ensure-output-dirs/defaults/main.yaml

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+zuul_output_dir: "{{ ansible_user_dir }}/zuul-output"

roles/ensure-output-dirs/tasks/main.yaml

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+- name: Ensure Zuul Output directories exist
+  file:
+    path: "{{ zuul_output_dir }}/{{ item }}"
+    state: directory
+  with_items:
+    - logs
+    - artifacts
+    - docs