Add option to choose the scan_tree() implementation in the NetApp backend

[achimnol]

In some customer setups, running the NetApp's XCP executable is too slow, incurring 2-3 seconds of delays whenever storage-proxy tries to scan the directory contents (list of files) regardless of how many files are there.

Since directory listing is a frequently invoked operation in the storage folder management UI, it should be fast.

Unfortunately it seems not easy to reduce the XCP invocation latency. Let's add an opt-out flag (e.g., netapp_xcp_exclude_scan_tree) to the NetApp storage plugin's configuration about whether to use XCP-based scan_tree() implementation for XCPFSOpModel or the vanilla scan_tree() implementation from the super-class, BaseFSOpModel.