Adapt fetch-fernet-tokens script to Python 3

Backport: train
Closes-Bug: #1859047
Change-Id: I9032e43bffa3a3de6c33a4ab59f65bafb0129dd5
(cherry picked from commit 902f16c)

Author: mnasiadka

docker/keystone/keystone-fernet/fetch_fernet_tokens.py

@@ -43,7 +43,7 @@ def has_file(filename_path):
 
 
 def num_tokens():
-    _, _, files = os.walk(TOKEN_PATH).next()
+    _, _, files = next(os.walk(TOKEN_PATH))
     return len(files)
 
 

releasenotes/notes/bug-1859047-d41762357da8ae0b.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fix bug which caused Keystone Fernet key distribution to fail on Python 3 systems,
+    by adapting fetch-fernet-keys.py script to work on Python 3.
+    `LP#1859047 <https://launchpad.net/bugs/1859047>`