Add role to move docs and artifacts to log root

When we're operating in the context of a change, we'd like to
upload artifacts and docs to the log server so that people can iterate
on them before publication safely. Add a role which is intended to be
run after fetch-output that moves the content on the executor so that
normal log upload concepts can be applied.

Change-Id: I1791d1283a0c1f112958db8861526145abe1357c

roles/merge-output-to-logs/README.rst

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+Put artifacts and docs into the executor log dir
+
+.. note::
+
+  This role only works in a trusted context. It is intended to
+  be used in the post playbook of a base job.
+
+This role moves artifacts and docs into the logs dir when
+``zuul.change`` is defined so that they can be uploaded to the
+log server for developer preview and validation.
+
+Artifacts and docs are left in place when ``zuul.change`` is
+not defined so that normal publication jobs can publish them
+to final locations.

roles/merge-output-to-logs/tasks/main.yaml

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+- name: Move artifacts and docs to logs dir
+  when: zuul.change is defined
+  delegate_to: localhost
+  shell: |
+    if ! $(ls {{ zuul.executor.work_root }}/{{ item }}) ; then
+      # Only create target directory if it is needed.
+      # Do not fail if it is already there.
+      mkdir -p {{ zuul.executor.log_root }}/{{ item }}
+      # Leave the original directory behind so that other roles
+      # operating on the interface directories can simply no-op.
+      mv -f {{ zuul.executor.work_root }}/{{ item }}/* {{ zuul.executor.log_root }}/{{ item }}
+    fi
+  loop:
+    - artifacts
+    - docs