Do not mount /var/lib/nova/mnt unless nfs cinder backend is enabled

When enable_cinder_backend_nfs is only used, /var/lib/nova/mnt is no
need to mount. Then we do not need care whether this folder is sharable.

Change-Id: I53f4c2c9ec25775cdb02a3256fd3a878723d15f6
Closes-Bug: #1644602