Add lower-constraints job to current development branch

The lower-constraints test was removed because of an issue where pip
could not correctly determine the required packages versions to install,
ending in an almost infinite loop that would end up in timeout, failure,
and general mayhem.
Recently the issue has been fixed and, if properly configured, the
lower-constraints test can provide good indication of which minimum
versions are required to support the current code.
This patch adds the test back to the current development branch, and it
runs only on master.
The lower-constraints file will stay in the future stable branches.

Change-Id: Ib28a19a930bb95a328f5b2d4c80a84126cb8b963

lower-constraints.txt

@@ -0,0 +1,14 @@
+Flask==1.0.2
+coverage==4.0
+ironic-lib==4.6.1
+libvirt-python==6.0.0
+munch==2.1.0
+openstacksdk==0.13.0
+oslotest==3.2.0
+pbr==2.0.0
+python-subunit==1.0.0
+requests==2.14.2
+stestr==1.0.0
+tenacity==6.2.0
+testscenarios==0.4
+testtools==2.2.0

zuul.d/project.yaml

@@ -1,9 +1,10 @@
 - project:
     templates:
-      - openstack-python3-xena-jobs
       - check-requirements
-      - release-notes-jobs-python3
+      - openstack-lower-constraints-master-branch-jobs
+      - openstack-python3-xena-jobs
       - publish-openstack-docs-pti
+      - release-notes-jobs-python3
     check:
       jobs:
         - sushy-tools-tempest-partition-bios-redfish-pxe