add lower-constraints job

Create a tox environment for running the unit tests against the lower
bounds of the dependencies.

Create a lower-constraints.txt to be used to enforce the lower bounds
in those tests.

Add openstack-tox-lower-constraints job to the zuul configuration.

See http://lists.openstack.org/pipermail/openstack-dev/2018-March/128352.html
for more details.

Change-Id: Ia0547a12c756388dc521c5eed7897140fe0dfd61
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

.zuul.yaml

@@ -8,6 +8,7 @@
         - watcher-tempest-workload_balancing
         - watcherclient-tempest-functional
         - legacy-rally-dsvm-watcher-rally
+        - openstack-tox-lower-constraints
     gate:
       jobs:
         - watcher-tempest-zuulv3
@@ -17,6 +18,7 @@
         - watcher-tempest-workload_balancing
         - watcherclient-tempest-functional
         - legacy-rally-dsvm-watcher-rally
+        - openstack-tox-lower-constraints
 
 - job:
     name: watcher-tempest-dummy_optim