[SPARK-17329][BUILD] Don't build PRs with -Pyarn unless YARN code changed

## What changes were proposed in this pull request?

Only build PRs with -Pyarn if YARN code was modified.

## How was this patch tested?

Jenkins tests (will look to verify whether -Pyarn was included in the PR builder for this one.)

Author: Sean Owen <sowen@cloudera.com>

Closes #14892 from srowen/SPARK-17329.