attach more security groups to load balancer

[shinenelson]

Oftentimes, there might be some extra rules that one might want to add to the load balancers.

Since the security_groups argument for resource "aws_lb" accepts a list, we could accept another list and then merge that with what we currently have.

Use case : I want to run an application on a custom port ( other than the default 80 and 443 ).

I am aware that it would be aesthetically better to run the load balancer on the default ports and then run the application on whatever different ports I want ( using container_port ). However, I have a hard requirement from a customer that the domain for the application use a different port than the default.