Facter facts via YAML fact source

This significantly speeds up MCollective inventory since there is no
longer any need to run Facter on every invocation. It implements the
ideas described in
http://projects.puppetlabs.com/projects/mcollective-plugins/wiki/FactsFacterYAML
for managing YAML facts from Facter. If $manage_yaml_facter_source==true
and $yaml_facter_source=='yaml' then on every Puppet run Facter facts
will be put into $yaml_facter_source (by default
/etc/mcollective/facts.yaml), excluding those that are highly dynamic,
such as uptime or memoryfree.