feat(contrib/ec2): always deploy into VPC; support all instance types

This commit rewrites our CloudFormation template for EC2. This includes
the following changes:
* Create a new VPC by default; existing VPC support is left as-is
* Support all EC2 instance types
* Support both PV- and HVM-virtualized AMIs

This also eliminates the 'No default VPC for this user' error that was
the result of not having a default VPC in the region.