ipa-client-install: add support for sss_ssh_knownhosts
    
    sss_ssh_knownhostsproxy will be deprecated in favor of sss_ssh_knownhosts.
    
    With this update, if the file /usr/bin/sss_ssh_knownhosts is present,
    KnownHostsCommand will be used instead of ProxyCommand. Also, GlobalKnownHostsFile
    is disabled as it is no longer needed.
    
    Fixes: https://pagure.io/freeipa/issue/9536
    Signed-off-by: Francisco Trivino <ftrivino@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>