ipalib: remove hard dependency on ipapython
    
    Hard-code the path to /bin/false in SubprocessError doc string.
    
    Remove ipaplatform dependency from ipalib's setup.py and add it as optional
    installer dependency to ipalib's and ipaclient's setup.py.
    
    https://fedorahosted.org/freeipa/ticket/6474
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>