Rework sensu_silenced name handling

[treydock]

The name passed like this:

      sensu_silenced { 'test':
        ensure => 'present',
        subscription => 'entity:sensu_agent',
      }

Will be replaced in metadata with entity:sensu_agent:*. The name passed during creation isn't used, and based on docs name is a #{subscription}:#{check}. To avoid confusion and inconsistent behavior the name of a sensu_silenced resource should match the convention that one would see when looking at things through sensuctl.

[treydock]

Instead of changing naming it was just documented how the naming works and tests added to validate what is documented. Closing via #1041.