'Given a username, set the user that plays are run by when using become'

def do_become_user(self, arg):

    if arg:
        self.options.become_user = arg
    else:
        display.display('Please    specify    a    user,    e.g.    `become_user    jenkins`')
        display.v(('Current    user    is    %s' % self.options.become_user))
    self.set_prompt()
