Make storage.utils.keyify py3k compatible

In the storage.utils.keyify function, there is proxy object, that wraps
each item and makes it comparable via __cmp__ function. Since Python 3
old-fashion `cmp` is totally removed and only rich comparison functions
should be used (i.e. __lt__, __gt__).

Partially-implements: blueprint py3k-support
Change-Id: I36841fa33c7e49b8b14fb623d6859077e3d8b6f5
Merge "Make storage.utils.keyify py3k compatible"