[SPARK-4095][YARN][Minor]extract val isLaunchingDriver in ClientBase

Instead of checking if `args.userClass` is null repeatedly, we extract it to an global val as in `ApplicationMaster`.

Author: WangTaoTheTonic <barneystinson@aliyun.com>

Closes #2954 from WangTaoTheTonic/MemUnit and squashes the following commits:

13bda20 [WangTaoTheTonic] extract val isLaunchingDriver in ClientBase