elbs:
  - name: "{{ e_d_c }}-public"
    scheme: "internet-facing"
  - name: "{{ e_d_c }}-internal"
    scheme: "internal"

If an internal elb exists, we need private subnets

This is functionally identical to the service_subnets and assumes you've
defined a private_elb_subnet_{1,2} similar to the existing
public_subnet_{1,2} variables (and requires code added in edx-secure's edx_service)

The subnet if statement is horride, but registering that variable as a
temporary global wasn't working.

Services might not have elbs, might only have public ELBs or might have
internal ELBs with no subnets configured.  Try to catch these cases
before ansible/ec2 give a less useful error message.