Allow specifying the subnet and VPC ID
Allow specifying the subnet and VPC ID

This makes this template into something we want to actually wrap instead of
having users use directly.  Next step is to create a template that creates a
new VPC, with bastions and subnets, and in turn calls this template with the
appropriate parameters.

Signed-off-by: Ken Simon <d689d7adec1f250ae6789ab10f904ed792f264cd@gmail.com>

heptio/aws-quickstart,captainshar/aws-quickstart,heptio/aws-quickstart