(maint) reduce log level duplication

Prior to this commit the interactive Puppet applications, `apply`,
`inspect`, `resource` had duplicate logic for setting log level based
on command line options.  This commit consolidates that logic into a
method in `Puppet::Application`.
Merge pull request #1981 from ericalthatcher/maint/master/reduce_duplication_for_setting_log_levels

(maint) reduce log level duplication