[SPARK-17810][SQL] Default spark.sql.warehouse.dir is relative to local FS but...

[SPARK-17810][SQL] Default spark.sql.warehouse.dir is relative to local FS but can resolve as HDFS path

## What changes were proposed in this pull request?

Always resolve spark.sql.warehouse.dir as a local path, and as relative to working dir not home dir

## How was this patch tested?

Existing tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #15382 from srowen/SPARK-17810.