add public IP address option for instances in VPC

allows you to pass :associate_public_ip_address to InstanceCollection#create, and applies this value to the network interface options.
NOTE: in order for this to work, a couple of other options have to be moved out of the base and into the network interface: private_ip_address, subnet, and security_group_ids
related to: https://forums.aws.amazon.com/thread.jspa?threadID=137014&tstart=0