Fix constraint jobs branch

neutron-docs-constraints and neutron-coverage-constraints have been
running on all branches - and also we have neutron-docs and
neutron-coverage running only on stable/kilo.

So, currently both kind of jobs are run on stable/kilo.
From the layout output:
  <Job neutron-docs> ^(?:stable/kilo).*$
  <Job neutron-docs-constraints>
  <Job neutron-coverage> ^(?:stable/kilo).*$
  <Job neutron-coverage-constraints>

Change the existing constraints job to not only cover gate jobs but also
these post jobs.

Change-Id: Ic1f370968aa93c6abbcf7667fe757037987f96d1

zuul/layout.yaml

@@ -2051,10 +2051,7 @@ jobs:
     branch: ^(?!stable/(?:juno|kilo)).*$
 
   # Constraints Jobs
-  - name: ^gate-.*-constraints$
-    branch: ^(?!stable/kilo).*$
-
-  - name: ^.*-tarball-constraints$
+  - name: ^.*-constraints$
     branch: ^(?!stable/kilo).*$
 
   - name: gate-neutron-python27