Explicitly decode decrypted secrets for py3

Being valid UTF-8 is already a requirement since we JSON encode these
secrets when sending them through gearman. It does raise the question
of whether we can or should support binary cleartext.

Change-Id: Iccf9fba9e1c07eff102965821dbf32bf2e1e7d36
diff --git a/zuul/configloader.py b/zuul/configloader.py
index ee663d5..50f5d16 100644
--- a/zuul/configloader.py
+++ b/zuul/configloader.py
@@ -158,7 +158,8 @@
         return cls(node.value)
 
     def decrypt(self, private_key):
-        return encryption.decrypt_pkcs1_oaep(self.ciphertext, private_key)
+        return encryption.decrypt_pkcs1_oaep(self.ciphertext,
+                                             private_key).decode('utf8')
 
 
 class NodeSetParser(object):