[bugfix] wrong client arg, should use executor-cores

client arg is wrong, it should be executor-cores. it causes executor fail to start when executor-cores is specified

Author: Tianshuo Deng <tdeng@twitter.com>

Closes #138 from tsdeng/bugfix_wrong_client_args and squashes the following commits:

304826d [Tianshuo Deng] wrong client arg, should use executor-cores