Ability to specify security groups by id

I just got stemcell to launch an instance. It's a very clever tool! :sailboat: 

A useful addition would be the ability to specify security groups via id as well as name. This patch enables that.

Related Ruby SDK documentation:
http://docs.aws.amazon.com/AWSRubySDK/latest/AWS/EC2/InstanceCollection.html#create-instance_method