zuul: arm64 pipeline

ARM64 testing is of interest to a number of projects.  For example:

 https://review.opendev.org/662456 : kolla
 https://review.opendev.org/676111 : diskimage-builder
 https://review.opendev.org/690798 : system-config (mirror tests)

However, we have been hesitant to put these tests into the main check
queue because with only a small number of nodes available (8
currently) it may lead to long delays merging code.

While jobs could go in the on-demand experimental pipeline, this
doesn't quite reflect the status of the jobs and nodes which are
operational.

I think that a separate queue to allow these jobs to run
automatically, but asynchronously to the regular check queue is a good
option here.  This will be a step to build a strong track-record for
the jobs without affecting the main gate, which should hopefully lead
to more interest and potentially more resources.

Change-Id: I77aac13521049496d43758121788aa44f0ecc990

zuul.d/pipelines.yaml

@@ -318,3 +318,32 @@
     # Don't report merge-failures to github
     merge-failure:
       mysql:
+
+# NOTE(ianw) 2019-12 : since we have very limited ARM64 resources, we
+# have a separate pipeline so ARM64 jobs can start and queue gerrit
+# changes automatically but don't hold up the main check pipeline.  Of
+# course, if we have more resources we can remove this and move jobs
+# back into the regular check/gate.
+- pipeline:
+    name: check-arm64
+    description: Pipeline for ARM64 based jobs.
+    success-message: Build succeeded (ARM64 pipeline).
+    failure-message: Build failed (ARM64 pipeline).
+    manager: independent
+    precedence: low
+    require:
+      gerrit:
+        open: True
+        current-patchset: True
+    trigger:
+      gerrit:
+        - event: patchset-created
+        - event: change-restored
+        - event: comment-added
+          comment: (?i)^(Patch Set [0-9]+:)?( [\w\\+-]*)*(\n\n)?\s*recheck
+    success:
+      gerrit: {}
+      mysql:
+    failure:
+      gerrit: {}
+      mysql: