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