[nova] Create for all client and non-client lib xena branch

This creates the stable/xena branch for all client
and non-client libraries now that we are past the
lib freeze on September 02.

If a team wants to wait for a specific patch to make it to the library,
someone from the team can -1 the patch to have it held,or update that
patch with a different commit SHA.

Patches with no response will be merged by Friday, September 10.

Change-Id: I3751fd2a3b3e43e57bcc7d5190eaae7996dbd68e

deliverables/xena/os-vif.yaml

@@ -16,3 +16,6 @@ releases:
     projects:
       - repo: openstack/os-vif
         hash: b837c1a74f37191692a820711e431a75516a4abf
+branches:
+  - name: stable/xena
+    location: 2.6.0

deliverables/xena/osc-placement.yaml

@@ -23,3 +23,6 @@ releases:
     projects:
       - repo: openstack/osc-placement
         hash: 0ea8f1b05d4aa4a6f144adff08771f5b84bda479
+branches:
+  - name: stable/xena
+    location: 3.1.1

deliverables/xena/python-novaclient.yaml

@@ -15,3 +15,6 @@ releases:
     projects:
       - repo: openstack/python-novaclient
         hash: 8066f8c745054caeb7bd66d4c2be15ab5fac1d1f
+branches:
+  - name: stable/xena
+    location: 17.6.0