[Tests] Fix unexpected tox output of SomeError exception
[Tests] Fix unexpected tox output of SomeError exception

This suppresses traceback of SomeError exception raised in
tests.unit.deployment.test_engine.EngineTestCase:

    Traceback (most recent call last):
      File "tests/unit/deployment/test_engine.py", line 198,
      in context_with_error
	raise error
    SomeError

This output does not impact tests result but clogs
tox messages

Change-Id: I78ca06ec33c971129185389bc9a6d6321718817d

eayunstack/rally,vganapath/rally,openstack/rally,vganapath/rally,vganapath/rally,gluke77/rally,eonpatapon/rally,eonpatapon/rally,eayunstack/rally,gluke77/rally,openstack/rally,yeming233/rally,eayunstack/rally,openstack/rally,yeming233/rally,vganapath/rally,gluke77/rally,openstack/rally,gluke77/rally