Fix PyCrypto imports on Case-Insensitive filesystems

PyCrypto fails to import when running in a case insensitive filesystem (eg. Vagrant NFS mount on OSX). 
Under that scenario, python tries to import the local crypto modulo instead of Crypto.  Using absolute_import is a way around that issue.

Here's a similar issue on the OpenStack Nova project:  
https://bugs.launchpad.net/nova/+bug/925792