CDH-22322: impala-shell script should set PYTHON_EGG_CACHE

This patch sets the impala-shell's PYTHON_EGG_CACHE to a per-user temporary location. Not
doing this can sometimes lead to permission issues.

Change-Id: I6dda335e3b2a91b4d471f8794bed8c351d90c9ae
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5311
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins