mailutils: fix for send_email() error on DEV site

* For the DEV site `send_email()` function was failing while trying to
  concatenate list and string (because the CFG_SITE_ADMIN_EMAIL
  was not a list).  (closes #1744)

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Reviewed-by: Yoan Blanc <yoan.blanc@cern.ch>