Allow option to avoid loading imfile module twice.

With version 8 of rsyslog we get warnings such as:
'module 'imfile' already in this config, cannot be added  [try http://www.rsyslog.com/e/2221 ]'.

This is because the imfile module should only be loaded once. If we have multiple imfile's by default we will always use a template which has the $ModLoad imfile statement in  (see https://github.com/saz/puppet-rsyslog/blob/master/templates/imfile.erb).

The rsyslog documentation also explains that the imfile module should only be loaded once: http://www.rsyslog.com/doc/imfile.html . This commit addresses this problem.