Passkey: extract the passkey from stdout
    
    SSSD's command passkey_child was previously using stderr to
    print the following messages:
    PIN required.
    Please touch the device.
    but switched to stdout instead in the commit
    https://github.com/SSSD/sssd/commit/6b0d175f8fad63c9c41e0b990c4a5632e96cc3f7
    
    Stdout was used only for displaying the generated passkey.
    
    This means that ipa user-add-passkey --register now must read
    stdout line by line and print only the messages that the user
    needs to see (all lines except the one containing the passkey).
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>