Prefer timeout to time_limit for Facter::Core::Execution

Facter 4 documentation and code was inconsistent regarding naming of
this option. limit and time_limit have been replaced by timeout
which also match the option name in previous versions of facter. The
old names will output a warning when used, so swith to the new
terminology now.

Releated PR:
https://github.com/puppetlabs/facter/pull/2429