Move Qpsmtpd::Config::get_qmail_map() to config/qmail_cdb plugin

The plugin itself seems to be working, however there's a test failure which I think exposes a problem that already existed.  It looks like config_dir() has been returning /var/qmail/control/ in the test suite in master when it should return ./config.sample/ (or ./t/config/ when QPSMTP_DEVELOPER is set).  This means that load_logging() has not found any configured logging plugins to find.  This branch fixed config_dir()'s behavior in the test suite, so now load_logging() finds config.sample/logging, and so it tries to load the logging/warn plugin.  However, Qpsmtpd::plugin_dirs() is being called by load_logging even though Test::Qpsmtpd::plugin_dirs() is defined.  Test::Qpsmtpd::plugin_dirs() would properly return 'plugins/', but Qpsmtpd::plugin_dirs() returns 't/plugins', leading to a failure to load logging/warn.  I haven't been able to figure out why this is happening...