client: move custom env variable into client module
    
    There is no need to have env as parameter because this is used only
    once, so it can eb safely moved to client.py module
    
    NOTE: PATH should be overwritten to safe values before we execute any
    command
    https://www.securecoding.cert.org/confluence/display/c/ENV03-C.+Sanitize+the+environment+when+invoking+external+programs
    
    https://fedorahosted.org/freeipa/ticket/6392
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>