AWS security groups are unique by name only by VPC (Restated, the VPC
and group name form a unique key).
When attaching security groups to an ELB, the ec2_elb_lb module would
erroneously find security groups of the same name in other VPCs thus
causing an error stating as such.
To eliminate the error, we check that we are attaching subnets (implying
that we are in a VPC), grab the vpc_id of the 0th subnet, and filtering
the list of security groups on this VPC. In other cases, no such filter
is applied (filters=None).
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