bug 842385 - update zamboni-stage configuration for sentry

- this is a big configuration change, I've copied all the settings
  from sites/altdev/* over and tried to keep all the staging area's
  domain settings.  Please review all domain name changes carefully to
  make sure I haven't botched anything
- metlog specific changes relevant to 842385 are contained in:
  * sites/stage/settings_mkt.py:line 54 to set django-statsd to use
    metlog as transport for statsd
  * sites/stage/settings_mkt.py:line 148-162 to configure the metlog
    client and instantiate it