Change vpc_subnet_id name to avoid recursive template substitution.

playbooks/edx-east/edx_provision.yml

@@ -21,7 +21,7 @@
     dns_zone: sandbox.edx.org
     name_tag: sandbox-temp
     elb: false
-    vpc_subnet_id: subnet-cd867aba
+    ec2_vpc_subnet_id: subnet-cd867aba
   roles:
     - role: launch_ec2
       keypair: "{{ keypair }}"
@@ -34,7 +34,7 @@
       dns_name: "{{ dns_name }}"
       dns_zone: "{{ dns_zone }}"
       zone: "{{ zone }}"
-      vpc_subnet_id: "{{ vpc_subnet_id }}"
+      vpc_subnet_id: "{{ ec2_vpc_subnet_id }}"
       assign_public_ip: yes
       terminate_instance: true
       instance_profile_name: sandbox