21832: Adds 'use_rds' TF var to create additional VPC resources.

21832: Adds 'use_rds' TF var to create additional VPC resources.

Even though an RDS instance supposedly can be set up as single-AZ,
it requires to be related to a DB Subnet Group resource, which in turn
requires at least 2 subnets from different AZs.
So, when enabling 'use_rds' this code creates a new subnet on a different
AZ from the one that private_subnet is in.
It also supports specifying a custom additional subnet id.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas.dipentima@curii.com>