Fix Invalid pathname for netapp cmode driver

In different Data ONTAP API versions exists incompatibility using
'nfs-exportfs-append-rules-2' API call.
8.2 requires field 'pathname' equal to volume's 'junction_path',
but 8.2.1 requires 'pathname' with prefix '/vol/' to volume's 'junction_path'.
So, make workaround for backend API trying second variant if first failed.

Change-Id: I63b70a3fe0895a1306c2b2c60d706965b141a184
Closes-Bug: #1365151