Add overwrite_hostname configuration parameter

Allow module to set overwrite_hostname parameter in gmond configuration.
Enables ganglia to overwirte erronious hostnames found for clients
reporting to server from a private network.

Changes:
	modified:   README.md
	modified:   manifests/gmond.pp
	modified:   templates/gmond.conf.debian.erb
	modified:   templates/gmond.conf.el6.erb