Enable Downstream trigger job in the architecture repo

In order to test VA HCI related changes, we used to run testproject in
the downstream.

downstream-va-hci-trigger-job drops the need of running testproject for
VA HCI related changes.

An developer can trigger the job by adding `trigger github-experimental`
in the pull request comment section. The trigger job will create a
review on
https://review.rdoproject.org/r/q/project:tripleo-downstream-trigger-nested-virt
repo with proper depends-on with the origin pr and dependendent change.

From tripleo-downstream-trigger-nested-virt gerrit review, it will start
VA HCI job downstream in rdo-check pipeline. Once the job finishes, It
will post the results on the gerrit review. Devs can verify the results
before merging it.

Signed-off-by: Chandan Kumar (raukadah) <raukadah@gmail.com>

zuul.d/projects.yaml

@@ -1,4 +1,14 @@
 - project:
+    github-experimental-trigger:
+      jobs:
+      # This pipeline will be triggered by adding `\trigger github-experimental`
+      # in the pull request comment.
+      - downstream-va-hci-trigger-job:
+          files:
+            - ^lib
+            - ^examples/common
+            - ^examples/dt/hci
+            - ^va/hci
     github-check:
       jobs: &id001
       - noop