add trusted hosts setting for gmetad, defaults to 127.0.0.1
adapt @maplebed's suggestion to leave this unset when the chef attr is unset.
default trusted_hosts to nil.
Update gmetad.conf.erb

match previous content
Merge pull request #28 from bitmonk/gmetad_trust

add trusted hosts setting for gmetad, defaults to 127.0.0.1