Fix "cannot import name abstract_driver" error in tox -e docs.
Fix "cannot import name abstract_driver" error in tox -e docs.

When doing tox -v -e docs, we got exception:
  File "/opt/stack/tacker/tacker/nfvo/drivers/vnffg/sfc_drivers/noop.py", line 20, in <module>
    from tacker.nfvo.drivers.vnffg.sfc_drivers import abstract_driver
ImportError: cannot import name abstract_driver

Change-Id: Ife9936ed7c2f9f6d621cd6f1949ec4211eec5b1e
Closes-bug: #1697710

openstack/tacker,stackforge/tacker,openstack/tacker,openstack/tacker,stackforge/tacker