Fix an instance where full standalone mode executor IDs were passed to
StandaloneSchedulerBackend instead of the smaller IDs used within Spark
(that lack the application name).

This was reported by ClearStory in
https://github.com/clearstorydata/spark/pull/9.

Also fixed some messages that said slave instead of executor.
Merge pull request #909 from mateiz/exec-id-fix

Fix an instance where full standalone mode executor IDs were passed to