go back to using separate lms-preview nginx app

We have still consolidated the unicorn processes, but we need separate
nginx configs for lms and lms-preview for preview to have basic auth.

playbooks/edx-west/prod-app.yml

@@ -40,6 +40,7 @@
     - role: nginx
       nginx_sites:
       - lms
+      - lms-preview
       - cms
       - verify
       nginx_template_directory: "{{local_dir}}/nginx/templates/"

playbooks/edx-west/stage-app.yml

@@ -35,6 +35,7 @@
     - role: nginx
       nginx_sites:
       - lms
+      - lms-preview
       - cms
       - verify
       nginx_template_directory: "{{local_dir}}/nginx/templates/"