Improve unit tests for glance.common package

Add tests for next modules:
* glance.common.wsgi
* glance.common.utils
* glance.common.client
* glance.common.config

Fixes: bug #1152183
Change-Id: Iedbdfb6af411170da10d078e48f5098f8d1c668b
Merge "Improve unit tests for glance.common package"