[SPARK-16106][CORE] TaskSchedulerImpl should properly track executors added to existing hosts

## What changes were proposed in this pull request?

TaskSchedulerImpl used to only set `newExecAvailable` when a new *host* was added, not when a new executor was added to an existing host.  It also didn't update some internal state tracking live executors until a task was scheduled on the executor.  This patch changes it to properly update as soon as it knows about a new executor.

## How was this patch tested?

added a unit test, ran everything via jenkins.

Author: Imran Rashid <irashid@cloudera.com>

Closes #13826 from squito/SPARK-16106_executorByHosts.