Don't use umount -R; CentOS 6 doesn't support it.

instead, unmount each directory separately.
ubuntu-headnode: don't create mocuser.

We're already setting the password for root, not mocuser, which is a
sensible enough approach -- but let's get rid of the superfluous
command, and document accordingly.
Merge pull request #370 from zenhack/ubuntu-headnode-cleanups

Ubuntu headnode cleanups