Add variable QoS to NetApp cDOT drivers

Add variable QoS to NetApp cDOT drivers

Most QoS implementations involve fixed limits, such as maxIOPS
or maxBPS.  But there are clouds that offer QoS limits that are
partly based on capacity of the underlying resource.  This commit
adds two new QoS flags to the NetApp cDOT drivers, maxIOPSperGiB
and maxBPSperGiB, which implement this capability.  A light
refactor of the snapshot delete paths was required to separate
those from the volume delete paths, since only the latter should
involve the QoS logic.

Implements: blueprint add-variable-qos-to-netapp-cdot-drivers
Change-Id: I357b87fa3a2856553c71c7f5f37e9f82ff44f6f2