Make sure unit tests set the correct log levels

Unit tests appear to not set the expected logging levels so
a number of modules will log excessivly (e.g. ISO8601).

This change ensures the base test classe sets the correct logging
levels (in the same way as the openstack.common.log module).

Change-Id: Ic11b7adee5f27f71094b4ef2b4a635f78f83cba8
Closes-Bug: #1340970
Merge "Make sure unit tests set the correct log levels"