Add `create_before_destroy` for parameter group. Make subnet group optional

aknysh commented 3 years ago

what

You cannot delete a parameter/option group while it is in use, but you can update a database with a new parameter/option group, so in order to change parameter/option groups, you need to create a new one, install it, then delete the old one
Some update and destroy operations were failing because resources were being deleted in the wrong order
If var.db_subnet_group_name is provided, use it and don't create a new Subnet Group
If var.availability_zone is provided (and var.db_subnet_group_name and var.subnet_ids are not), use var.availability_zone to place the instance into the default VPC or EC2 Classic