Shell scripts compatibility enhancements

Here are some changes in attempt to make the setup shell scripts more portable (in particular - compatible with Ubuntu 13.10).
* Use `.` instead of `source` to load `vars.sh`. Seems `source` is not POSIX compatible. (In particular - it's not part of `dash` shell which is the default shell mapped to `/bin/bash` in recent versions of Debian and Ubuntu). *(Another alternative would be to explicitly require some other shell like `bash` or `zsh` instead of `sh`)*
* Parameterize some parts of the variables: to allow easier override. (E.g. - my vbox dir is located elsewhere)
* Add some up-front checks of isos and boxes dir existence to avoid having partially configured vbox VMs.