Append nas_opts to IBMNAS_NFSDriver configuration

'nas_opts' were added in the NFS Driver (nfs.py), but were not appended
to the IBMNAS driver configuration, causing failure with the multi-backend
configuration of driver.

Adding the missing configuration.

Change-Id: I55727566d03573908750d01ae1345c4b6276f131
Closes-Bug: #1298279
Merge "Append nas_opts to IBMNAS_NFSDriver configuration"