Enable the `certs` NginX site on sandbox/silo config.

This fixes a startup error with the "certs" role: upon restart, the
"certs" supervisord service tries to contact the service through the
public port (which should be) served by NginX and fails.

playbooks/edx_sandbox.yml

@@ -22,6 +22,7 @@
   roles:
     - role: nginx
       nginx_sites:
+      - certs
       - cms
       - lms
       - forum