Enhance ec2 scripts, add vpc support

I made a few changes to ec2 scripts, they now support launching clusters inside specified VPC and subnet.
Also added support for placement groups, and a list of "secure ips" that are added to security groups so your cluster won't be open for a whole world.
Ephemeral disks were not always properly attached ( sometimes I got only two on cc2.8xlarge when I should have 4 ) - fixed that