Backport the native Zuul jobs to the stable branch

Squash together all the changes now that the jobs are stable enough.
The jobs (definitions and code) should be available in the stable
branches as well. This will be handled naturally in the future, when
stable/queens is branched from master.

Change-Id: Icf0e57cf948435cb2fee8d30b76369b6dba4c617

.zuul.yaml

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+- project:
+    name: openstack/sahara-image-elements
+    check:
+      jobs:
+        - sahara-image-elements-buildimages-ambari:
+            voting: false
+        - sahara-image-elements-buildimages-cloudera:
+            voting: false
+        - sahara-image-elements-buildimages-mapr:
+            voting: false
+        - sahara-image-elements-buildimages-plain:
+            voting: false
+        - sahara-image-elements-buildimages-spark:
+            voting: false
+        - sahara-image-elements-buildimages-storm:
+            voting: false
+        - sahara-image-elements-buildimages-vanilla:
+            voting: false
+
+- job:
+    name: sahara-image-elements-buildimages-basejob
+    timeout: 5400
+    run: playbooks/buildimages/run.yaml
+
+- job:
+    name: sahara-image-elements-buildimages-ambari
+    parent: sahara-image-elements-buildimages-basejob
+    vars:
+      sahara_plugin: ambari
+
+- job:
+    name: sahara-image-elements-buildimages-cloudera
+    parent: sahara-image-elements-buildimages-basejob
+    vars:
+      sahara_plugin: cloudera
+
+- job:
+    name: sahara-image-elements-buildimages-mapr
+    parent: sahara-image-elements-buildimages-basejob
+    vars:
+      sahara_plugin: mapr
+
+- job:
+    name: sahara-image-elements-buildimages-plain
+    parent: sahara-image-elements-buildimages-basejob
+    vars:
+      sahara_plugin: plain
+
+- job:
+    name: sahara-image-elements-buildimages-spark
+    parent: sahara-image-elements-buildimages-basejob
+    vars:
+      sahara_plugin: spark
+
+- job:
+    name: sahara-image-elements-buildimages-storm
+    parent: sahara-image-elements-buildimages-basejob
+    vars:
+      sahara_plugin: storm
+
+- job:
+    name: sahara-image-elements-buildimages-vanilla
+    parent: sahara-image-elements-buildimages-basejob
+    vars:
+      sahara_plugin: vanilla

playbooks/buildimages/run.yaml

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+- hosts: all
+  roles:
+    - build-sahara-images-dib

roles/build-sahara-images-dib/README.rst

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+Build Sahara Images with DIB
+
+**Role Variables**
+
+.. zuul:rolevar:: sahara_build_directory
+   :default: /var/tmp/sahara-image-build
+
+   The base directory used for the build process.
+
+.. zuul:rolevar:: sahara_plugin
+   :default: vanilla
+
+   The plugin whose images will be built.

roles/build-sahara-images-dib/defaults/main.yaml

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+---
+sahara_image_elements_src_dir: "{{ zuul.projects['git.openstack.org/openstack/sahara-image-elements'].src_dir }}"
+sahara_build_directory: /var/tmp/sahara-image-build
+sahara_plugin: vanilla

roles/build-sahara-images-dib/tasks/main.yaml

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+- name: Build all the images for the selected plugin
+  command: ./tools/gate/build-images "{{ sahara_plugin }}"
+  args:
+    chdir: "{{ sahara_image_elements_src_dir }}"