Integrate the new oslotest library

There is a new support library named 'oslotest'
extracted from oslo incubator. So, we add it to
test-requirements.txt and remove deprecated modules
from rally.openstack.common

Change-Id: I1d9efaf2f8a5a4948c454ea3f2e75262479133e8
Merge "Integrate the new oslotest library"