NetApp E-Series driver: Remove caching logic

NetApp E-Series driver: Remove caching logic

This patch removes unnecessary API caching logic from the NetApp
E-Series driver. The cache, represented as the _objects dictionary,
is refreshed nearly every API call and therefore provides no value.
This refactor is also preparation for refactoring the class structure
so we can easily add an E-Series Fibre Channel driver.

Change-Id: I5a1a8c9bab3abff58a876a1da3575b1c329fdfba