NetApp NFS efficient clone_image impl

NetApp NFS efficient clone_image impl

This change implements clone_image funcionality
for NFS drivers. It implements an internal image cache
in nfs shares which will be used to clone images in case
the image_location supplied does not represent an nfs share
which could be used to directly clone image. It also implements
an internal cache cleaning mechanism at regular interval.

blueprint netapp-cinder-nfs-image-cloning

Change-Id: I20eaddf9febc78df33e31bd41b4698b772da3ae7