Add lets_encrypt to role edx_sandbox.yml

Add lets_encrypt role to edx_sandbox, with conditional on
LETS_ENCRYPT_DOMAINS true (non-empty list).

playbooks/edx_sandbox.yml

@@ -33,6 +33,8 @@
       nginx_sites:
       - ecommerce
       when: SANDBOX_ENABLE_ECOMMERCE
+    - role: lets_encrypt
+      when: LETS_ENCRYPT_DOMAINS
     - role: edxlocal
       when: EDXAPP_MYSQL_HOST == 'localhost'
     - role: memcache