support gerrit AWS provisioning with cloud formation

Note:
* Sets up a VPC.  Is that overkill?
* Deploys the app server and the ELB into the public subnet, RDS into the private.  I considered putting the app server in the private subnet, but that would require also running a NAT/bastion host, which seemed like a lot of overhead for a single app server.
* Removes the nginx reverse proxy since the ELB can now server that purpose.
* Completely untested.

Reviewers: @e0d, @jarv, @gwprice