ipa: add SYSDB_USER_MAPPED_CERT for certs in idoverrides
    
    Recent changes to support a rule based mapping between users and
    certificates were not properly added for the case where a certificate
    was added to an idoverride for a user. As a result authentication with
    the certificate from the idoveride was not possible. With this patch the
    certificate from to idoveride is properly added to the cache.
    
    Related to https://pagure.io/SSSD/sssd/issue/3603
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>