forum-nginx-sites-playbook: Add missing forum config in nginx_sites

Some playbooks are missing the new forum nginx reverse proxy configuration
file, which prevents the LMS to access the forum service.

playbooks/edx_sandbox.yml

@@ -18,6 +18,7 @@
       nginx_sites:
       - cms
       - lms
+      - forum
       - ora
       - xqueue
       nginx_default_sites:

playbooks/vagrant-fullstack.yml

@@ -17,6 +17,7 @@
       - cms
       - lms
       - ora
+      - forum
       - xqueue
       nginx_default_sites:
       - lms