Clean up crypto.py
Clean up crypto.py

 * Remove chained certs since we don't use them
 * Make get_vpn use the existing generate call instead of shell script
 * (Bonus: we can revoke vpn certs now)

Change-Id: I8e118c5bd3dee6ba7c6a2a1390874b69008c436b

n0ano/ganttclient