N1kv: Fixes fields argument not None

N1kv plugin includes 'fields' in call to get_<resource> function in
superclass. This can trigger errors if other fields are used in the
subsequent processing in the N1kv plugin's own get_<resource>
function.

Change-Id: I63687a830579a178c6daaa2b8f71bef7eb53d584
Closes-bug: #1285289
Merge "N1kv: Fixes fields argument not None"