ipatests: fix test_ca_custom_sdn
    
    The test fails because it configures the server with a custom --subject-base
    and --ca-subject that contains a comma. The installation failure is a known
    issue 7347 that was fixed in ipa-4-7 but not in ipa-4-6 branch (the fix relies
    on recent version of 389-ds-base 1.4.0.20 that is not available on fedora 27).
    
    The fix: use a custom ca-subject and subject-base without any comma.
    
    Fixes: https://pagure.io/freeipa/issue/8126
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>