Add download artifact role

This role can be used to download a previously built artifact.
Requires a Zuul API endpoint.

Change-Id: I5dd88f67bfa383f5f2bc0cfcc3b2346def796ae7

roles/download-artifact/README.rst

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+Download an artifact from a completed build of a Zuul job
+
+Given a change downloads an artifact from a previous build (by default
+of the current change) into the work directory.
+
+**Role Variables**
+
+.. zuul:rolevar:: download_artifact_api
+
+   The Zuul API endpoint to use.  Example: ``https://zuul.example.org/api/tenant/{{ zuul.tenant }}``
+
+.. zuul:rolevar:: download_artifact_pipeline
+
+   The pipeline in which the previous build ran.
+
+.. zuul:rolevar:: download_artifact_job
+
+   The job of the previous build.
+
+.. zuul:rolevar:: download_artifact_name
+
+   The artifact name.
+
+.. zuul:rolevar:: download_artifact_query
+   :default: change={{ zuul.change }}&patchset={{ zuul.patchset }}&pipeline={{ download_artifact_pipeline }}&job_name={{ download_artifact_job }}
+
+   The query to use to find the build.  This should return exactly one
+   result.  Normally the default is used.

roles/download-artifact/defaults/main.yaml

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+---
+download_artifact_query: "change={{ zuul.change }}&patchset={{ zuul.patchset }}&pipeline={{ download_artifact_pipeline }}&job_name={{ download_artifact_job }}"

roles/download-artifact/tasks/main.yaml

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+- name: Query Zuul API for artifact information
+  uri:
+    url: "{{ download_artifact_api }}/builds?{{ download_artifact_query }}"
+  register: build
+- name: Extract artifact URL
+  set_fact:
+    archive_url: "{{ (build.json[0].artifacts | selectattr('name', 'equalto', download_artifact_name) | list)[0].url }}"
+- name: Download archive
+  uri:
+    url: "{{ archive_url }}"
+    dest: "{{ zuul.executor.work_root }}"