Make AWS region selectable on CLI

* Make it convenient (albeit, mandatory) to set AWS region on the
  command line when calling ansible-playbook for cloudformation

README.md

@@ -111,9 +111,10 @@ Example users are in the `vars/secure` directory:
 
   ```
   cd playbooks
-  ansible-playbook  -vvv cloudformation.yaml -i inventory.ini  -e 'key=<key_name> name=<stack_name> group=<group_name>'
+  ansible-playbook  -vvv cloudformation.yaml -i inventory.ini  -e 'region=<aws_region> key=<key_name> name=<stack_name> group=<group_name>'
   ```
   
+* _aws_region_: example: `us-east-1`. Which AWS EC2 region to build stack in.
 * _key_name_: example: `deploy`. SSH key name configured in AWS for the region
 * _stack_name_: example: `EdxAppCustom`. Name of the stack, must not contain
   underscores or cloudformation will complain. Must be an unused name or

playbooks/cloudformation.yaml

@@ -32,7 +32,7 @@
   - name: edX configuration
     cloudformation: >
       stack_name="$name" state=present
-      region=us-east-1 disable_rollback=false
+      region=$region disable_rollback=false
       template=files/edx-server-ubuntu-configuration.json
     args:
       template_parameters: