(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.
The [default loglevel is 2](https://github.com/puppetlabs/puppet/blob/master/lib/puppet/util/log.rb#L16) which corresponds to `notice`. For some reason, the `doc` application increases the default level to `warning`, which will cause messages at the `notice` level to be discarded. I'm not sure if that's was done intentionally, or just another puppet inconsistency.