BYO tags for azure

[alekssaul]

My customer uses Azure Tags for billing purposes i.e. they attach billTo=XYZ tags to resources to designate which dept to bill.

We forked terraforming-azure and have a azure_resource_tags variable that gets merged into existing tags such as account_for=bosh etc.

i.e.

  tags = {
    environment = "${var.env_name}"
    account_for = "bosh"
  }

becomes

  tags = "${merge(map(
    "environment", var.env_name,
     "account_for", "bosh"),
     var.azure_resource_tags
    )}"

I was wondering if I could file a PR to upstream this. I figured the use case can generally by applied.