Remove cinderclient/tests from coverage report

Currently coverage reports on coverage of the tests themselves. This
makes the percentage coverage higher than if they are excluded and
therefore distorts the reports values for the codes as a whole.

This commit updates the .coveragerc files to remove
cinderclient/tests/* from the coverage report.

Change-Id: If7f1765017f3f0d55975219b12ff7bf149146931
Merge "Remove cinderclient/tests from coverage report"