(PUP-2998): Make log_level settable in puppet.conf

Added a call hook to the `log_level` setting so that this setting can be
defined in `puppet.conf` in addtion to being passed as a command line flag.
Merge pull request #3131 from Sharpie/PUP-2998-fix-loglevel

(PUP-2998): Make log_level settable in puppet.conf