Update to use setup_extension

Update after the removal of _setUpExtension. [1]

[1] I8135fc4eb05e474db7224b7771b71270a93a47bf

This commit includes the following unrelated change
to pass the gate.
----------------------------------------
Fix cover job

Copy-and-paste from networking-midonet.

Closes-Bug: #1794509
Change-Id: I5ae3be917deb94673928a236662f953e4cd143ac
----------------------------------------

Closes-Bug: #1794457
Change-Id: Ia4d84c7f0dcea44e3131dd756853c003e9c8471d

neutron_taas/tests/unit/extensions/test_taas.py

@@ -35,10 +35,9 @@ class TaasExtensionTestCase(test_api_v2_extension.ExtensionTestCase):
 
     def setUp(self):
         super(TaasExtensionTestCase, self).setUp()
-        self._setUpExtension(
+        self.setup_extension(
             'neutron_taas.extensions.taas.TaasPluginBase',
             'TAAS',
-            taas_ext.RESOURCE_ATTRIBUTE_MAP,
             taas_ext.Taas,
             'taas',
             plural_mappings={}

tox.ini

@@ -32,10 +32,15 @@ commands = {posargs}
 
 [testenv:cover]
 basepython = python2.7
-commands = 
-  python setup.py test --coverage --coverage-package-name=neutron_taas --testr-args='{posargs}'
+setenv =
+  {[testenv]setenv}
+  PYTHON=coverage run --source neutron_taas --parallel-mode
+commands =
+  stestr run --no-subunit-trace {posargs}
+  coverage combine
   coverage report --fail-under=50 --skip-covered
-
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
 
 [testenv:docs]
 commands = python setup.py build_sphinx