[SPARK-10594] [YARN] Remove reference to --num-executors, add --properties-file

[erickt]

ApplicationMaster no longer has the --num-executors flag, and had an undocumented --properties-file configuration option.

cc @srowen

[vanzin]

ok to test

[vanzin]

LGTM although no one really should be executing this class directly.

[andrewor14]

Yup LGTM merging into master.

[SparkQA]

Test build #42440 has finished for PR 8754 at commit c7d1686.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.