Set better defaults for LMS_BASE, CMS_BASE

Having an empty CMS_BASE variable causes the "View Unit in Studio" button on the LMS
to not work properly. These are better defaults.

playbooks/roles/edxapp/defaults/main.yml

@@ -23,9 +23,9 @@ EDXAPP_DJFS:
   directory_root: '{{ edxapp_data_dir }}/django-pyfs/static/django-pyfs'
   url_root : '/static/django-pyfs'
 
-EDXAPP_LMS_BASE: ""
-EDXAPP_PREVIEW_LMS_BASE: ""
-EDXAPP_CMS_BASE: ""
+EDXAPP_LMS_BASE: "{{ EDXAPP_LMS_SITE_NAME }}:{{ EDXAPP_LMS_NGINX_PORT }}"
+EDXAPP_PREVIEW_LMS_BASE: "preview.{{ EDXAPP_LMS_SITE_NAME }}:{{ EDXAPP_LMS_PREVIEW_NGINX_PORT }}"
+EDXAPP_CMS_BASE: "{{ EDXAPP_CMS_SITE_NAME }}:{{ EDXAPP_CMS_NGINX_PORT }}"
 
 EDXAPP_LMS_GUNICORN_EXTRA: ""
 EDXAPP_LMS_GUNICORN_EXTRA_CONF: ""