Add stable branch jobs on the plugins master gate

Supported stable branches in maintenance state
(stein, train, ussuri) use the Tempest and plugins master
version for their testing. To make sure master version (new
tests or any changes) is compatible with stable branch testing,
we need to add the stable branches job on the plugins master gate.
Tempest and many other plugins are already testing the stable
branches in same way[1].

Ussuri onwards, Tempest or plugins are py3 only, so we are
running only python3 version of stable jobs

Tempest plugins guide for stable branch testing:
- https://docs.openstack.org/tempest/latest/stable_branch_testing_policy.html

 [1] e8f1876aa6/.zuul.yaml (L646)

Change-Id: Id7e69347f316fa79f60e7969fee300798397cf89

.zuul.yaml

@@ -2,5 +2,25 @@
     templates:
       - openstack-python3-ussuri-jobs
       - check-requirements
+    check:
+      jobs:
+        - ec2-api-functional-neutron-ussuri
+        - ec2-api-functional-neutron-train
+        - ec2-api-functional-neutron-stein
     gate:
       queue: ec2-api
+
+- job:
+    name: ec2-api-functional-neutron-ussuri
+    parent: ec2-api-functional-neutron
+    override-checkout: stable/ussuri
+
+- job:
+    name: ec2-api-functional-neutron-train
+    parent: ec2-api-functional-neutron
+    override-checkout: stable/train
+
+- job:
+    name: ec2-api-functional-neutron-stein
+    parent: ec2-api-functional-neutron
+    override-checkout: stable/stein