Get OpenID XRDS url host from request

Replaced the previous method of getting it from HTTP_POST to use
django's mechanism, which may or may not use HTTP_HOST. However if an
attacker changes the request header, there is not much he can do since
he cannot recreate the association nonce.

common/djangoapps/external_auth/views.py

@@ -271,10 +271,7 @@ def get_xrds_url(resource, request):
     """
     Return the XRDS url for a resource
     """
-    host = request.META['HTTP_HOST']
-
-    if not host.endswith('edx.org'):
-        return None
+    host = request.get_host()
 
     location = host + '/openid/provider/' + resource + '/'