Allow ami and instance type env vars (#303)
Allow ami and instance type env vars (#303)

Allows for environment variables to override
the ami and instance type used and give
these environment variables the highest precedence.

This is to allow us to perform builds from the
Jenknis GUI where we set these variables.

The SWAN_AMI variable is intended to support
building out an AMI snapshot from any given
AMI ID.
intelsdi-x/swan,intelsdi-x/swan,intelsdi-x/swan