bug 827461 - metlog for marketplace production

* added METLOG and METLOG_CONF configuration for sites/prod/settings_mkt.py
* added RAVEN_CONFIG to force registration of django signals into raven

The previous set of configuration only overrode the SENTRY_CLIENT setting to djangoraven.metlog.MetlogDjangoClient.  The MetlogDjangoClient won't do anything unless Metlog itself is enabled.