netapp plugin: Refactored reading of disk data.

Instead of obscuring control-flow with generic function pointers, use a
clear and easy to read function hierarchy. All disk-related action now
starts with “cna_query_disk (host)” instead of
“service->handler (host, data, service->data)”.

The “GetDiskPerfData”block has been renamed to “Disks”. All those
blocks start with “Get” and most end with “PerfData”, distracting
from the actual relevant part.

The “Multiplier” option has been replaced by the “Interval” option,
which expects a time in seconds rather than a factor which is multiplied
to the host interval.