api: deprecate the concept of extensions in v2.1

We want to remove the extension infrastructure in the v2.1 API during
the next release. To make that possible, we must deprecate the
configuration now.

Also deprecating the osapi_v3 group, and moving to osapi_v21.

UpgradeImpact
DocImpact

Part of blueprint nova-api-deprecate-extensions

Change-Id: I084444b11dceda7cf8f88c157aa67d36490fce49

nova/tests/functional/api_sample_tests/api_sample_base.py

@@ -54,7 +54,7 @@ class ApiSampleTestBaseV3(testscenarios.WithScenarios,
                 whitelist.update(set(self.extra_extensions_to_load))
 
             CONF.set_override('extensions_whitelist', whitelist,
-                              'osapi_v3')
+                              'osapi_v21')
         expected_middleware = []
         if (not hasattr(self, '_test') or (self._test == 'v2.1')):
             # NOTE(gmann)For v2.1 API testing, override /v2 endpoint with v2.1