(FACT-698) Add fips_enabled fact

Adds a fips_enabled fact to determine if FIPS is enabled on the system.

FACT-698
Historically we've had issues reading from `/proc` filesystems, e.g. for example the `filesystems` fact shells out to `cat` so that ruby doesn't block indefinitely. I'm honestly not sure if this still the case, or just lore from facter releases of old?

Also we might want to be using https://github.com/puppetlabs/facter/blob/master/lib/facter/util/file_read.rb#L25 to read the file contents.

@adrienthebo thoughts?