Add Zuul job variable to indicate a job will retry

This change adds a variable to post and cleanup playboks in order to
determine if a job will be retried due to a failure in one of the
earlier playbooks.

This variable might be useful for only performing certain actions (e.g.
interacting with a remote system) when the job result is final and there
won't be any further attempts.

Change-Id: If7f4488d4a59b1544795401bdc243978fea9ca86

releasenotes/notes/zuul-will-retry-job-variable-9a5a132cdc8591f0.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    A new Zuul job variable :var:`zuul_will_retry` available in post and
+    cleanup playbooks, which indicates whether the current job will be retried.