Warn user about unsupported API version

Prints a warning when the user tries to use an unsupported API.
Also provides a test case to check it still does fall back to 2.0.

Change-Id: Idabd150f52d27a9d065cf570f6367459126f98be
Closes-bug: #1256987

keystoneclient/shell.py

@@ -431,6 +431,10 @@ class OpenStackIdentityShell(object):
                 "2.0": shell_v2_0.CLIENT_CLASS,
             }[version]
         except KeyError:
+            if version:
+                msg = ('WARNING: unsupported identity-api-version %s, '
+                       'falling back to 2.0' % version)
+                print(msg)
             return shell_v2_0.CLIENT_CLASS
 
     def do_bash_completion(self, args):

keystoneclient/tests/test_shell.py

@@ -200,6 +200,11 @@ class ShellTest(utils.TestCase):
                       'wilma', 'betty', '2.0', True, '500', True)
             self.assertTrue(all([x == y for x, y in zip(actual, expect)]))
 
+            # Test os-identity-api-version fall back to 2.0
+            shell('--os-identity-api-version 3.0 user-list')
+            assert do_tenant_mock.called
+            self.assertTrue(b.os_identity_api_version, '2.0')
+
     def test_shell_user_create_args(self):
         """Test user-create args."""
         do_uc_mock = mock.MagicMock()