Control the subnet that an instance is started in
VPC config

* 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 branch 'ec2-subnet-id' of github.com:wjlroe/nixops into ec2-subnet-id

Conflicts:
	nixops/backends/ec2.py