Pass document root to Mustache

'Partial' templates (those that are referred to from other templates)
were not properly resolved relative to the working directory. This
change passes the webserver document root to the template library.

Change-Id: Ib1f4d72b86f33507214dfd14bd23593bf76a9857
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3724
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins