IMPALA-6216: Make PYTHON_EGG_CACHE configurable
IMPALA-6216: Make PYTHON_EGG_CACHE configurable

User can set environment variable PYTHON_EGG_CACHE before call
impala-shell.

Testing:
Run impala-shell with or w/o PYTHON_EGG_CACHE configured

Change-Id: I695b2b31d9045eef1a53268f6516858935aed508
Reviewed-on: http://gerrit.cloudera.org:8080/12911
Tested-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>
Reviewed-by: Bharath Vissapragada <f23dbca75acdac4082d6a05c290fc03565dada86@cloudera.com>
Reviewed-on: https://gerrit.sjc.cloudera.com/c/cdh/impala/+/49569
Tested-by: Jenkins User <d95b56ce41a2e1ac4cecdd398defd7414407cc08@cloudera.com>
Reviewed-by: Laszlo Gaal <57b770caf3b7d570ebf40813dac2ce5f53722fa7@cloudera.com>
CDH-Build: Laszlo Gaal <57b770caf3b7d570ebf40813dac2ce5f53722fa7@cloudera.com>
Quasar-L0: Laszlo Gaal <57b770caf3b7d570ebf40813dac2ce5f53722fa7@cloudera.com>

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