Python 3.6 compatibility: bytes vs strings stuff

These changes mostly revolve around the change in ObjectId constructor
when running on Python 3.6. Where on 2.7 the constructor would accept
12- and 24-byte strings, now only 12-byte bytes and 24-character strings
are accepted. Good thing, but required some changes in our code.

Other changes include hashing of strings, which isn't supported, so they
are converted to bytes first, and sometimes converted back afterwards.

pillar/api/utils/__init__.py

@@ -136,7 +136,7 @@ def str2id(document_id):
 
     try:
         return bson.ObjectId(document_id)
-    except bson.objectid.InvalidId:
+    except (bson.objectid.InvalidId, TypeError):
         log.debug('str2id(%r): Invalid Object ID', document_id)
         raise wz_exceptions.BadRequest('Invalid object ID %r' % document_id)