Terraform: Delay the MySQL Provider
I think you could achieve something similar by first running Terraform against your RDS resources, and then running Terraform again on all resources.

On your first pass, you can use -target option to target only a subset of the resources. (ie. RDS resources). 

Be aware that using this option is not recommended in Terraform docs. If I were you, I would use it for time being and change it as soon as one of these issues below is solved:
https://github.com/hashicorp/terraform/issues/10462
https://github.com/hashicorp/terraform/issues/2430
https://github.com/hashicorp/terraform/issues/4149

For details on how to use -target and its caveats, see: https://www.terraform.io/docs/commands/plan.html#resource-targeting