oauthclient: local account discovery improvement

• Uses userEXT table to do a lookup of the external identifier in order to find the local account corresponding to the user that logs in using oauth. (closes #2532)

Signed-off-by: Javier Martin Montull <javier.martin.montull>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Reviewed-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>