Add StackForge Libra tarball jobs.

PBR was just integrated into Libra, so we'd like to take
advantage of that.

Change-Id: I5b1e54ffb28324c1a06691300d1963f74ad9974b
Reviewed-on: https://review.openstack.org/26521
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins

modules/openstack_project/files/zuul/layout.yaml

@@ -1033,6 +1033,12 @@ projects:
     gate:
       - gate-libra-pep8
       - gate-libra-python27
+    post:
+      - libra-branch-tarball
+    pre-release:
+      - libra-tarball
+    release:
+      - libra-tarball
 
   - name: stackforge/python-libraclient
     check: