Fix incorrect error message after running
Fix incorrect error message after running

The script would say `[INFO] Verify NFS mount ... FAIL` even though it
was perfectly fine.

The reason was that `docker-machine ssh <machine-name> df` returned
only: `exit status 1`

This seems to be related to some aufs-related file systems where df
gets a "permission denied".

I fixed this by making the df command always return with exit status 0.

mstrzele/minikube-nfs,adlogix/docker-machine-nfs