Use non greedy aws_iam_user_group_membership (#12)
aws_iam_group_membership [1] is a greedy resource that can cause
inconsistent behaviour when adding a users in multiple places. The
resource will conflict with itself if used more than once with the same
group. To non-exclusively manage the users in a group switch to the
aws_iam_user_group_membership [2] resource which can be used multiple
times with the same user for non-overlapping groups.
Note that terraform-aws-iam-user [3] is using the
aws_iam_user_group_membership resource. This mix is causing issues when
running root account level Terraform. `users` is run and any subsequent
runs of `iam` (which calls this module) is greedily removing users from
groups that were associated in the `users` run.
[1] https://www.terraform.io/docs/providers/aws/r/iam_group_membership.html
[2] https://www.terraform.io/docs/providers/aws/r/iam_user_group_membership.html
[3] https://github.com/cloudposse/terraform-aws-iam-user/blob/master/main.tf#L20-L25
