import zuul job settings from project-config

This is a mechanically generated patch to complete step 1 of moving
the zuul job settings out of project-config and into each project
repository.

Because there will be a separate patch on each branch, the branch
specifiers for branch-specific jobs have been removed.

Because this patch is generated by a script, there may be some
cosmetic changes to the layout of the YAML file(s) as the contents are
normalized.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Also, fix tools/tox_install.sh:
build-openstack-sphinx-docs jobs fail on stable branches due to the new
pip version 10 introduces some previously Warning as Error: in case of
calling "pip install" without any package name, the command fails.
tox_install.sh is called during docs job without any package passed to pip.

Change-Id: I9b937eb5bbef968ce53460dd4267a57bf33d013e
Story: #2002586
Task: #24288

.zuul.yaml

@@ -0,0 +1,15 @@
+- project:
+    templates:
+      - openstack-python-jobs
+      - openstack-python35-jobs
+      - publish-openstack-sphinx-docs
+      - check-requirements
+      - lib-forward-testing
+      - release-notes-jobs
+    check:
+      jobs:
+        - openstack-tox-pylint:
+            voting: false
+    post:
+      jobs:
+        - openstack-tox-cover

tools/tox_install.sh

@@ -26,5 +26,10 @@ pip install -c"$localfile" openstack-requirements
 # install will be constrained and we need to unconstrain it.
 edit-constraints "$localfile" -- "$CLIENT_NAME"
 
+if [ -z "$@" ]; then
+    echo "No packages to be installed."
+    exit 0
+fi
+
 pip install -c"$localfile" -U "$@"
 exit $?