NetApp NFS and iSCSI: move zapi client logic into modules

This patch moves the logic for constructing zapi requests
into its own modules in order to reduce coupling with
driver logic, improve testability, and improve readability.
This patch also adds unit tests around the zapi request
logic.

Implements bp improve-netapp-drivers

Change-Id: I3939df9a55d77b14d723422c25bd3dd3bcef9fbe
Merge "NetApp NFS and iSCSI: move zapi client logic into modules"