Setup specified 'work_dir' using sudo - though only if needed, shouldn't cause any installs that were working without sudo to suddenly need it.

Amazon instances seem to have less root than my OpenStack instances so I ran out of space while installing Galaxy-P because of the large amount of space required by OpenMS. I can now just add 'work_dir = /mnt/cblwork' to fabricrc to get around this by using ephemeral disk. In theory, ephemeral disk should also be faster - though in practice but I haven't actually observed this.