Adds v3 API disable config option

Adds the ability to disable the v3 API and it is disabled by
default for now. The API is explictly enabled when running the
unittests though.

Change-Id: I34597d35b9739e06561579651f7ec72afa357919

nova/test.py

@@ -227,6 +227,7 @@ class TestCase(testtools.TestCase):
         self.useFixture(fixtures.EnvironmentVariable('http_proxy'))
         self.policy = self.useFixture(policy_fixture.PolicyFixture())
         CONF.set_override('fatal_exception_format_errors', True)
+        CONF.set_override('osapi_v3_enabled', True)
 
     def _clear_attrs(self):
         # Delete attributes that don't start with _ so they don't pin