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>