Allow launching instances in existing subnets

This was specifically requested as something that users would want,
ahead of full VPC support. This is also something that the existing
Cloud Provisioner does not do.

@mrzarquon if you have time to check if this works for your use case that would be much appreciated.
FYI, should be `ec2_client(region)`, otherwise this causes most tests relying on instance creation to bomb.

I have this fixed already in an updated PR, but just a heads up.