Include phase name in log for some phases

Phases createiso, liveimages, image_build, ostree_installer and osbs are
done in parallel, logs from these phases are mixed and and it's not
obvious which log message belongs to which phase. This change adds phase
name in log message for these phases.

The new mixin 'PhaseLoggerMixin' is added to extend a Pungi phase with a
logging logger which copy handlers from compose's logger but with
formatter changed.

Fixes: #58
Signed-off-by: Qixiang Wan <qwan@redhat.com>

bin/pungi-koji

@@ -181,7 +181,8 @@ def main():
 
     from pungi.compose import Compose
 
-    logger = logging.Logger("Pungi")
+    logger = logging.getLogger("Pungi")
+    logger.setLevel(logging.DEBUG)
     kobo.log.add_stderr_logger(logger)
 
     conf = kobo.conf.PyConfigParser()