In order to optimize the NetApp ONTAP driver, this patch is caching
the status of driver pools and reusing for the each share server,
given that the pool is not separated by share server.
The option `netapp_cached_aggregates_status_lifetime` is added
for controlling the time that the cached values is considered
valid.
Closes-Bug: #1900469
Change-Id: I14a059615fc29c7c173c035bb51d39e0bbb8b70a