Fix for rhbz #1412260
Return None in _open_cert when ~/.fedora.upn exist is not correct,
i.e. we can read cert if we delete ~/.fedora.upn ?
With latest commits we already have enough warnings that fedora does
not use client certificate anymore, but if other external repos try to use this
code, the code works instead have an traceback error.
Signed-off-by: Sérgio M. Basto <sergio@serjux.com>