Asymmetric vault: validate public key in client
    
    The ipa vault commands now load and validate the public key for
    asymmetric encryption, before sending it to the server. This prevents
    invalid vaults and prohibits accidental exposure of private key
    material.
    
    https://fedorahosted.org/freeipa/ticket/5142
    https://fedorahosted.org/freeipa/ticket/5143
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>