Updated inconsistent value of scope parameter

For explicit unscoped authentication, the keystome server
specifies the scope parameter value as 'unscoped'. However
keystoneauth1 initialises it to {'unscoped': {}}. This
patch removes this inconsistency.

Updated the corresponding test for unscoped request which
checks the scope parameter in auth to match the 'unscoped'
string.

Partial-Bug #1637682

Change-Id: I1c9d89fd86773f4acecbefcdad4dca6cff16b58d

keystoneauth1/identity/v3/base.py

@@ -154,7 +154,7 @@ class Auth(BaseAuth):
         elif self.trust_id:
             body['auth']['scope'] = {'OS-TRUST:trust': {'id': self.trust_id}}
         elif self.unscoped:
-            body['auth']['scope'] = {'unscoped': {}}
+            body['auth']['scope'] = 'unscoped'
 
         # NOTE(jamielennox): we add nocatalog here rather than in token_url
         # directly as some federation plugins require the base token_url

keystoneauth1/tests/unit/identity/test_identity_v3.py

@@ -550,7 +550,7 @@ class V3IdentityPlugin(utils.TestCase):
         self.assertEqual(token.user_id, ident['password']['user']['id'])
         self.assertEqual(password, ident['password']['user']['password'])
 
-        self.assertEqual({}, body['auth']['scope']['unscoped'])
+        self.assertEqual('unscoped', body['auth']['scope'])
 
     def test_unscoped_with_scope_data(self):
         a = v3.Password(self.TEST_URL,