Use fact() function for all os.distro.* facts

Use fact() function for all os.distro.* facts

* On Puppet 6 facter 3.x requires lsb-release to resolve os.distro.* facts. Using $facts hash cause errors like "Evaluation Error: Operator '[]' is not applicable to an Undef Value." because os.distro is undefined causing the catalog to fail. Use fact() to identify Undef facts and throw an error to the user.

Signed-off-by: Christos Papageorgiou <christos.papageorgioy@gmail.com>