'rcmd: run a command on underlying server
in root namespace
args: string or list of strings
returns: stdout and stderr'

def rcmd(self, *cmd, **opts):

    popen = self.rpopen(*cmd, **opts)
    result = ''
    while True:
        poll = popen.poll()
        result += popen.stdout.read()
        if (poll is not None):
            break
    return result
