[SPARK-4096][YARN]let ApplicationMaster accept executor memory argument in same format as JVM memory strings

Here `ApplicationMaster` accept executor memory argument only in number format, we should let it accept JVM style memory strings as well.

Author: WangTaoTheTonic <barneystinson@aliyun.com>

Closes #2955 from WangTaoTheTonic/modifyDesc and squashes the following commits:

ab98c70 [WangTaoTheTonic] append parameter passed in
3779767 [WangTaoTheTonic] Update executor memory description in the help message