Add ARN attribute to aws_elb resource

[joshuaspence]

It seems that the aws_elb resource doesn't output an arn attribute (whereas the aws_lb resource does). The use case for this is a custom Lambda function that we have written which takes a list of domains to generate certificates for using LetsEncrypt, as well as a list of ELB ARNs to use for updating certificates.

[bflad]

This is similar to #2261 for aws_ebs_volume where the API doesn't directly provide it, but we can manually construct it. PR incoming.

[bflad]

Submitted PR: #2272

[ghost]

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