Add possibility to remove graphite_prefix and graphite_suffix from config file

[ttarczynski]

I need a possibility to remove options graphite_prefix and graphite_suffix from config file (and from graphite metric names).

With current version from master when I didn't set the graphite_prefix param, I've got this in config file:

:graphite_prefix: undef
:graphite_suffix: puppet
:graphite_reverse_hostname: true

and also got undef in graphite metric names.

---

After this change:

• prefix is not added by default
• both prefix and suffix can be removed by setting them to 'absent':

class { '::graphite_reporter':
  graphite_prefix => 'absent',
  graphite_suffix => 'absent',
}