Deployment object comes from Rally framework and it has special
`get_credentials_for` method. For backward compatibility, it transforms
raw dict credentials to OpenStackCredential object.
This compatibility level should be removed someday and this day has come
since we moved rally_openstack.credential module to
rally_openstack.common.credential place and `get_credentials_for`
produces warning now.
Change-Id: I84f3c366ce23b6e67089d7ce053ca0f88d795b01
8de6f3d230