Purging maas-rack-controller crashes when removing authbind configuration

[Gavin Panella]

maas-rack-controller.postrm contains:

        # Remove authbind
        MAAS_UID="`id -u maas`"
        if [ -f "/etc/authbind/byuid/$MAAS_UID" ]; then
            rm -rf /etc/authbind/byuid/$MAAS_UID
        fi

However, the `maas` user may not exist by the time this is called.