[SPARK-9731] Standalone scheduling incorrect cores if spark.executor.cores is not set

The issue only happens if `spark.executor.cores` is not set and executor memory is set to a high value.
For example, if we have a worker with 4G and 10 cores and we set `spark.executor.memory` to 3G, then only 1 core is assigned to the executor. The correct number should be 10 cores.
I've added a unit test to illustrate the issue.

Author: Carson Wang <carson.wang@intel.com>

Closes #8017 from carsonwang/SPARK-9731 and squashes the following commits:

d09ec48 [Carson Wang] Fix code style
86b651f [Carson Wang] Simplify the code
943cc4c [Carson Wang] fix scheduling correct cores to executors