fixes #137 - Better support for non-hostname certnames

This patch introduce a new setting :use_uuid_for_certificates which
defaults to false.

users who wish to deploy their hosts using a random unique id, should
enable that setting, and use the certname directive in their puppet.conf
template, e.g.:
certname = <%= @host.certname %>

when the certname was not generated (e.g. for hosts created before this
patch) @host.certname would return the host name instead.

for provisioning where DNS is not reliable, it is highly recommended to
use certnames.

this patch enables future work, to identify systems (while being
provisioned) via UUID, not requiring the need for ipaddress/mac based
detection, and therefore avoiding NAT and similar issues.