Convert bytes to string in get_id for python3 compatibility

base64.b32encode() will return bytes in python3, let's convert
it to string.

Partially-Implements: blueprint magnum-python3

Change-Id: I8c9920e29e090f6b68aa95943484bea6af937eb3

magnum/common/short_id.py

@@ -51,7 +51,10 @@ def get_id(source_uuid):
     # The first 12 bytes (= 60 bits) of base32-encoded output is our data
     encoded = base64.b32encode(six.b(random_bytes))[:12]
 
-    return encoded.lower()
+    if six.PY3:
+        return encoded.lower().decode('utf-8')
+    else:
+        return encoded.lower()
 
 
 def generate_id():