Server list from environment

If the environment variable BEANSTALK_SERVERS is set, supplement the list of servers with its content.
There were warnings being generated for variables being unassigned, specifically `$groups` by `lib/tpl/modalFilterColumns.php`, included by `lib/tpl/main.php`.  `lib/tpl/main.php` doesn't set these like `lib/tpl/allTubes.php` and `lib/tpl/currentTube.php` do.  Seemed wise to always set them.