Removes unused imports in the oslo.config.tests module

Change-Id: Ibf4d9a697432eb0644f7a0e763ab2f78d9fd8505
Adds pylint section in tox.ini in oslo.config to check for unused imports

Fixes bug #1157597

Change-Id: I58057cdde6064dacf6689560ec6439fd6dc7cf4f