(MAINT) Add --debug option to `apply_manifest_on` helper

Before every 'puppet apply' command is executed with --verbose.
Now you can pass `debug: true`, which executes 'puppet apply' with the
--debug flag.
Merge pull request #442 from leoc/add-debug-flag-to-apply-manifest-on

(MAINT) Add --debug option to `apply_manifest_on` helper