(PUP-3624) Refactor log setup

Previously, the cert, doc, and face based applications had the same
logic for checking if `--verbose` or `--debug` was specified on the
command line, and setting the log level appropriately.

This commit refactors those applications to just call the
`Puppet::Application#set_log_level` method instead. The doc application is
a little bit unique in that it changes the default log level from
`notice` to `warning`, and it explicitly sets a console log destination.
(PUP-3624) Log at least at info level when --verbose is specified

The change for PUP-2998 means if you have `log_level=debug` in your
puppet.conf, then `puppet agent --test` will reduce the log level to info,
because `--test` implies `--verbose`.

This commit changes the semantics of `--verbose` (and `--test`) so that
we log *at least* at that level, but we won't downgrade.  This affects
all Puppet::Applications, e.g. puppet apply, agent, etc.

Note this wasn't an issue before PUP-2998 was implemented, because
log_levels could only be specified on the command line.
Merge pull request #3346 from joshcooper/ticket/master/PUP-3624_log_at_least_debug

(PUP-3624) Log at least at info level when --verbose is specified