[SPARK-18967][SCHEDULER] compute locality levels even if delay = 0

## What changes were proposed in this pull request?

Before this change, with delay scheduling off, spark would effectively
ignore locality preferences for bulk scheduling.  With this change,
locality preferences are used when multiple offers are made
simultaneously.

## How was this patch tested?

Test case added which fails without this change.  All unit tests run via jenkins.

Author: Imran Rashid <irashid@cloudera.com>

Closes #16376 from squito/locality_without_delay.