Add a API test list sanity check gate job to Zuul

This patch set adds a simple Zuul gate job that lists the plugin's
API tests as a sanity check.

Change-Id: I8462002b7b13ba760a25b629e2aff78b0cebb4bc

.zuul.yaml

@@ -13,3 +13,19 @@
 - project:
     templates:
       - tempest-plugin-jobs
+    check:
+      jobs:
+        - tox-list-api-tests
+    gate:
+      jobs:
+        - tox-list-api-tests
+
+- job:
+    name: tox-list-api-tests
+    parent: tox
+    nodeset: ubuntu-xenial
+    description: |
+      Use tox to run a stestr list against the tests/api/ directory as a
+      sanity check
+    vars:
+      tox_envlist: listapitests

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = pep8,py35,py27
+envlist = pep8,py35,py27,listapitests
 skipsdist = True
 
 [testenv]
@@ -25,6 +25,11 @@ basepython = python3
 commands = flake8 {posargs}
            check-uuid --package airship_tempest_plugin.tests.api
 
+[testenv:listapitests]
+basepython = python3
+commands = stestr init
+           stestr --test-path ./airship_tempest_plugin/tests/api list
+
 [testenv:uuidgen]
 basepython = python3
 commands = check-uuid --package airship_tempest_plugin.tests.api --fix