(FACT-599) Don't hardcode location of `sysctl`

`sysctl` is found in `/sbin` on OpenBSD but is in `/usr/bin` on OSX.
Because we can't assume the location it's easier to allow
`Facter::Core::Execution.which` to locate the executable to invoke.
Merge pull request #704 from adrienthebo/bugfix/facter-2/fact-599-sysctl-path

(FACT-599) Don't hardcode location of `sysctl`