Convert base64 encoded value to string

If we leave the value has as the result of b64decode(), we end up with
a bytes-like object that gets serialized into our zuul.conf as
b'.....', which is likely not what we actually want.  Instead, assume
this is a utf-8 encoded value and decode it that way.

Change-Id: Ie61ad82c022888000e72bedfea99123dda34d5c0

zuul_operator/zuul.py

@@ -213,7 +213,7 @@ 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,