(#23373) Support selecting the section

The --section option to the subcommand now allows a specific section of
the configuration file to be manipulated.

This allows both "puppet config set" and "puppet config print" to control which
section on the configuration file they are dealing with. As a side effect of
getting that functionality, there is now an interface for reading different
parts of the configuration information, independent of what "run mode" might be
in effect.