openidcp: allow setting default attribute mapping at install
    
    This was not wired up to the install script, so it could only
    be set by the IPA helper. In some circumstances it might be nice
    to set it explicitly on the command line (e.g. when setting up
    a Bodhi development environment without using IPA).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>