pylint: Fix format-string-without-interpolation
    
    Found by new Pylint:
    > ipaclient/install/client.py:1926:
    [W1310(format-string-without-interpolation), get_ca_certs] Using
    formatting for a string that does not have any interpolated variables)
    
    Fixes: https://pagure.io/freeipa/issue/9117
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>