Fix forwarded hostname for pycon servers

It appears (https://github.com/caktus/pycon/issues/188) that requests
getting to the backend production servers are being identified to
Django as having been addressed to the staging server - Django is
picking up the hostname as staging-pycon.python.org instead of
us.pycon.org.

I'm guessing this is due to the wrong value for proxy_pass in the
nginx config. On production I see:

  proxy_pass http://staging-pycon.python.org;

I'm also guessing I can set the application name in PyCon's app.rb
to correct this.

Once this is corrected, Django's ALLOWED_HOSTS settings will need
to accept the new hostname, so I've updated that as well. (It's
okay for that to include all the possible hostnames.)
Pass along host header as suggested by dstufft
Merge pull request #47 from python/pycon-forwarded-host-name

Fix forwarded hostname for pycon servers
Merge branch 'master' into env-for-all-commands