Add project templates for openstack-tox-cover

Currently many projects use it [1] and in many cases adding requirement
of neutron or horizon. Currently there are other templates that covers
this need for horizon and neutron for other base jobs (e.g. py3*).

This patch adds the following templates for the openstack-tox-cover job:

  - openstack-cover-jobs
  - openstack-cover-jobs-horizon
  - openstack-cover-jobs-neutron

[1]:
http://codesearch.openstack.org/?q=-%20openstack-tox-cover&i=nope&files=&repos=

Change-Id: I5feb6bb43385784c534f56390252b7f740390a70

zuul.d/project-templates.yaml

@@ -456,6 +456,38 @@
             required-projects:
               - openstack/neutron
 
+- project-template:
+    name: openstack-cover-jobs
+    description: |
+        Runs openstack-tox-cover in only the check pipeline.
+    check:
+      jobs:
+        - openstack-tox-cover
+
+- project-template:
+    name: openstack-cover-jobs-horizon
+    description: |
+      The standard openstack-cover-jobs but with horizon added as a
+      required-projects. This checks out the project code and allows
+      tox_install_siblings to use it when building the venv.
+    check:
+      jobs:
+        - openstack-tox-cover:
+            required-projects:
+              - openstack/horizon
+
+- project-template:
+    name: openstack-cover-jobs-neutron
+    description: |
+      The standard openstack-cover-jobs but with neutron added as a
+      required-projects. This checks out the project code and allows
+      tox_install_siblings to use it when building the venv.
+    check:
+      jobs:
+        - openstack-tox-cover:
+            required-projects:
+              - openstack/neutron
+
 - project-template:
     name: openstack-python35-jobs
     check: