Code review comment for lp:~salgado/launchpad/bug-530738

We're currently passing the application URL as the realm when starting
an OpenID authentication. That means when you login on, say,
bugs.lp.net, the provider will be told that the realm is bugs.lp.net,
but that's not really true as the authentication is valid for all of
launchpad.net. That is also how the spec says the realm should be used:

  A realm is designed to give the end user an indication of the scope
  of the authentication request.
  (http://openid.net/specs/openid-authentication-2_0.html#realms)

This branch fixes that by always using the mainsite's root URL as the
realm (aka trust root).