[SPARK-16606][CORE] Misleading warning for SparkContext.getOrCreate "WARN...

[SPARK-16606][CORE] Misleading warning for SparkContext.getOrCreate "WARN SparkContext: Use an existing SparkContext, some configuration may not take effect."

## What changes were proposed in this pull request?

SparkContext.getOrCreate shouldn't warn about ignored config if

- it wasn't ignored because a new context is created with it or
- no config was actually provided

## How was this patch tested?

Jenkins + existing tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #14533 from srowen/SPARK-16606.