This commit fixes#8264 by making the security_groups attribute on
aws_alb resources computed, allowing the default security group assigned
by AWS to not generate perpetual plans forcing new resources.
This commit adds a resource, acceptance tests and documentation for the
new Application Load Balancer (aws_alb). We choose to use the name alb
over the package name, elbv2, in order to avoid confusion.
This is the first in a series of commits to fully support the new
resources necessary for Application Load Balancers.