This will allow for configurations that use other front-ends for
authentication. One example might be mod_auth_mellon for SAML SSO support
(which sets MELLON_NAME_ID).

Testing Done:
Not sure how to inject `settings` in `test_x509_auth_middleware tests` to test
"CUSTOM". Tested on a copy of our reviewboard server which uses mod_auth_mellon
and references our SSO server. Works like a charm!

Reviewed at https://reviews.reviewboard.org/r/10131/