Revert "Fix failing unit tests"

Nova-team already reverted commits, which change exceptions in novaclient
whithout backward compatibility, so we should revert our local fix, which
adapted usage of exceptions for novaclient v 2.18.0 .

This reverts commit 895f3cbc55be0cce2b831093610e5addb3c50f58.

Change-Id: I728a434d2578a7148569eb7be459709fa47a73d4