Add a 'defaults' plugin to enable common collectd plugins.

Useful if you override the whole config file.
Retrieves the current collectd version with facter and configures only
plugins available in the installed collectd version.
Support pre-4.7 network configs.

Requires the facter modules to retrieve the collectd version.
Allow to set all write_graphite options.

Also keep the defaults from the current implementation,
allowing to override them.