Decode operator secrets as strings

The secret values need to be decoded as strings, otherwise they will be
written to zuul.conf as:

  foo=b'BAR'

which prevents them from working properly

Change-Id: I42ce0775ff0fd2906d5515e348a84858c0f1329e

zuul_operator/zuul.py

@@ -212,7 +212,7 @@ class Zuul:
                     if k == 'sshkey':
                         v = f'/etc/zuul/connections/{connection_name}/sshkey'
                     else:
-                        v = base64.b64decode(v)
+                        v = base64.b64decode(v).decode('utf-8')
                     connection[k] = v
 
         kw = {'connections': connections,