Feature request: Support tagging of ELB and GATEWAY through terraform

[arvindsree]

Created from #88 on behalf of @infa-apatel

[jeremyodle]

It would be great if ALL Aviatrix resources supported tagging where the underlying AWS resources supported them. I would also include descriptions if possible. We utilize tagging for our clients to manage multiple use cases including (but not limited to): 1) billing and cost management, 2) inventory management, 3) DevOps/automation, 4) security, and 5) IAM controls.

I'd also suggest implementing a map(string) instead of a list(string) for the tag type. This way you can utilize merge() functions without having to transform typical tag maps into the Aviatrix resources (e.g. tag_list = compact(distinct([for tag, value in local.tags : format("%s:%s", tag, value)])) vs. tags = local.tags).