[New feature] Add feature to reset_token (logout)

Currently, the CLI only has functionality to `set_token` and `get_token`. However, the `set_token` only accepts 40-character long strings. Therefore, once a token is added, there is no way to take that back expect to switch it with a different token. Also, there is functionality to `evalai login` but no `logout`.
Expected: There should be a command to log out. And also one for resetting token to a undefined value.