Add PR template with instructions for repo members

this will serve as a reminder that sometimes Jenkins jobs can be
missing or failing and also lists the comments which team members
can use to trigger Jenkins jobs, especially for PRs from
non-members.

(cherry picked from commit de6419f0d1)

.github/pull_request_template.md

@@ -0,0 +1,19 @@
+--- Description of proposed changes ---
+
+
+
+
+--- Merge policy ---
+
+- [ ] Travis CI PASS
+- [ ] `*-aws-runtest` PASS
+- [ ] `*-azure-runtest` PASS
+- [ ] `*-images-runtest` PASS
+- [ ] `*-openshift-runtest` PASS
+- [ ] `*-vmware-runtest` PASS
+
+--- Jenkins commands ---
+
+- `ok to test` to accept this pull request for testing
+- `test this please` for a one time test run
+- `retest this please` to start a new build