Fix incorrect error message after running

[sbesselsen]

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". Stuff like this:

none                     18.2G      4.2G     13.0G  24% /mnt/sda1/var/lib/docker/aufs/mnt/d52aab1e7fec612742470b048dc9baf4887c8c52dba430d106a0e72e426f502f
df: /var/run/docker/netns/d52aab1e7fec: Permission denied

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

[ajmath]

cool

[tonivdv]

Thank you @sbesselsen