[SPARK-10952] Only add hive to classpath if HIVE_HOME is set.

Currently if it isn't set it scans `/lib/*` and adds every dir to the
classpath which makes the env too large and every command called
afterwords fails.

Author: Kevin Cox <kevincox@kevincox.ca>

Closes #8994 from kevincox/kevincox-only-add-hive-to-classpath-if-var-is-set.