Add neutron as required-projects

We are trying to support firewall group logging in Rocky cycle.  There
are some patches be depended on neutron master branch which is not
released along with setted milestone yet.  This causes following jobs
failed and blocked the progress of the development.

    * openstack-tox-lower-constraints
    * build-openstack-sphinx-docs

This patch intends to consume master branch of neutron for above jobs.
On the other hand, neutron and neutron-fwaas are released together. So
there is no need to care lower bound of neutron.

Co-Authored-By: Yushiro FURUKAWA <y.furukawa_2@jp.fujitsu.com>
Change-Id: I93136aec69083ce43ebfe151245a3077941b88bc

.zuul.yaml

@@ -4,10 +4,22 @@
         - neutron-fwaas-networking-midonet-cross-py35:
             voting: false
 
-        - openstack-tox-lower-constraints
+        - openstack-tox-lower-constraints:
+            required-projects:
+              - openstack/neutron
+
+        - build-openstack-sphinx-docs:
+            required-projects:
+              - openstack/neutron
     gate:
       jobs:
-        - openstack-tox-lower-constraints
+        - openstack-tox-lower-constraints:
+            required-projects:
+              - openstack/neutron
+
+        - build-openstack-sphinx-docs:
+            required-projects:
+              - openstack/neutron
 - job:
     name: neutron-fwaas-networking-midonet-cross-py35
     parent: openstack-tox-py35