TOTP auth not functional in python3

Fixing a byte>str conversion bug present in the TOTP
passcode generation function that was only present in python3
which rendered TOTP auth non-functional in python3.

Also adding a test to check passcode generation returns the
correct format.

Closes-Bug: #1607119

Change-Id: Ie052d559c4eb2577d35caa9f6e240664cf4cf399

keystone/auth/plugins/totp.py

@@ -62,7 +62,7 @@ def _generate_totp_passcode(secret):
     decoded = base64.b32decode(secret)
     totp = crypto_totp.TOTP(
         decoded, 6, hashes.SHA1(), 30, backend=default_backend())
-    return six.text_type(totp.generate(timeutils.utcnow_ts(microsecond=True)))
+    return totp.generate(timeutils.utcnow_ts(microsecond=True)).decode('utf-8')
 
 
 @dependency.requires('credential_api')

keystone/tests/unit/test_v3_auth.py

@@ -16,6 +16,7 @@ import copy
 import datetime
 import itertools
 import operator
+import re
 import uuid
 
 from keystoneclient.common import cms
@@ -5070,6 +5071,12 @@ class TestAuthTOTP(test_v3.RestfulTestCase):
 
         self.v3_create_token(auth_data, expected_status=http_client.CREATED)
 
+    def test_generated_passcode_is_correct_format(self):
+        secret = self._make_credentials('totp')[-1]['blob']
+        passcode = totp._generate_totp_passcode(secret)
+        reg = re.compile(r'^-?[0-9]+$')
+        self.assertTrue(reg.match(passcode))
+
 
 class TestFetchRevocationList(object):
     """Test fetch token revocation list on the v3 Identity API."""