Domain fact cannot be resolved on travis without ffi

https://github.com/puppetlabs/facter/pull/2315 changed the way we resolve hostname on domain, also relying on FFI.
When FFI is not installed, Facter will not continue to search for hostname and domain and it will not resolve the facts.

This was noticed in puppetserver-ca-cli pipeline.

A fix was done in https://github.com/puppetlabs/facter/pull/2318 but this is not sufficient.
If FFI cannot be loaded, we must log a debug message and continue the search.