allow parametrization of repo files through config variables #10

Right now, my own configuration packages are parametrized at build time. Various credentials and moving parts (e.g. network interface names) are interpolated as parameters.

It would be nice to have a mechanism like that in Holo itself, so that e.g. configuration variables could be read e.g. from /etc/holo. This would allow hologram packages to be independent from machine-specific credentials, thus enabling easier sharing of holograms between users.

The system should also be built in such a way that fact collection (such as Facter or Ohai) and fact distribution (such as etcd) can be integrated easily.

(Migrated from holocm/holo-attic#10.)