Map from log app-name to an application.

def apptags(self):
        """
        Map from log app-name to an application.
        """
        logger.debug("populate tags map ...")
        apps = self._apps.keys()
        unknown = set(apps)
        unknown.difference_update(self._config_apps.keys())
        if unknown:
            raise ValueError("unknown apps: %r" % list(unknown))

        apps = [v for v in self._config_apps.values() if v.name in apps]
        tagmap = {}
        for app in sorted(apps, key=lambda x: (x.priority, x.name)):
            for tag in app.tags:
                if not tag:
                    raise LogRaptorConfigError('found an empty tag for app %r' % app.name)
                try:
                    tagmap[tag].append(app)
                except KeyError:
                    tagmap[tag] = [app]
        return tagmap