Refactor NetApp Cmode driver

7mode driver is expected to be removed as obsolete, so we need remove any
dependency on it. Cluster Mode driver depends on it.

Changes:
- rewritten some methods
- added methods, that were used from 7mode driver
- used abc.metaclass for helpers
- updated related unittests

Partially implements blueprint remove-netapp-7mode-driver

Change-Id: I3f720fa36c5b602c079a880fa7f9b2e95864385d