Implement get_volume_stats in NFS driver

Use the already existing code (ie: _get_available_capacity) to
implement get_volume_stats in the nfs driver

Change the netapp nfs driver to use this new implementation
instead of the dumb one.

Fixes bug #1152501

Change-Id: Ie16d9628ba4b096c60966b4aa6192fbdcb9a4f20
Merge "Implement get_volume_stats in NFS driver"