ipa-advise: fallback to /usr/libexec/platform-python if python3 not found
    
    when ipa-advise generates a script to configure a client for smart card
    auth, the script calls python3 to configure SSSD. The issue happens
    if the server (when ipa-advise is run) and the client do not have
    the same path for python3 command.
    
    By default, try to use python3 but if the command is not found, fallback
    to /usr/libexec/platform-python (which is the python3 path on RHEL8).
    
    Fixes: https://pagure.io/freeipa/issue/8311
    Reviewed-By: Christian Heimes <cheimes@redhat.com>