provider/aws: Provide iops when changing storage type to io1 on RDS

[Bowbaq]

iops is required when specifying a storage_type of io1. In most cases, iops will also have changed when storage_type changes, but not always. For example:

resource "aws_db_instance" "instance" {
  ...

  allocated_storage = "100"
  storage_type      = "gp2"
  iops              = "1000"
}

then

resource "aws_db_instance" "instance" {
  ...

  allocated_storage = "100"
  storage_type      = "io1"
  iops              = "1000"
}

[catsby]

Thanks @Bowbaq – I imagine this will be refactored in something like #5679 but for now this looks good!

[ghost]

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