Control the subnet that an instance is started in

* Specify the Subnet that an instance is in
* Specify SecurityGroupIds (you can't use security group names with the
  non-default VPC)
* Specify whether instance should have a public IP assigned (by default
  in the non-default VPC this is not the case)
Merge pull request #217 from HealthUnlocked/ec2-subnet-id

Control the subnet that an instance is started in