Elasticache vpc security groups

A couple of changes to `ec2_elb_lb` and `elasticache` to add better ec2 security group support.

For `elasticache`, only cache security groups which allow public access to elasticache instances.  I added a `security_group_ids` option to allow for internal ec2 applications access to elasticache (like inside a vpc).

For `ec2_elb_lb`, I added a `security_group_ids` options to allow for optionally setting one or more security groups to your elb.