NetApp 7mode NFS driver doesn't honor netapp_vfiler option

This patch fixes the NetApp 7mode NFS driver to register and use
the netapp_vfiler option if it is configured in cinder.conf.

DocImpact
Change-Id: I4db42d2521d7e6018f4f7ad0c4ab13441871675e
Closes-Bug: 1381716
(inspired by commit: 504cb9f3d9954138fcb62d2fbb20b8505ca0f0ac)