Allow for more flexibility in VPC configuration

[mattdelsordo]

Right now, you can specify VPC settings for your Tilegarden deployment by including info in your .env file. However, if you want to make your Tilegarden instance one chunk of a larger AWS-based system that all gets deployed at once, you may not know these values at deploy-time. It should be possible to rig up some means of generating these values automatically, or at least have them be automatically provided to Tilegarden's deployment scripts by another part of the deployment process.