Rewrite scripts/install_gluster.sh so that it can reuse the functions defined in scripts/helper_functions.sh

Certain functions in `scripts/install_gluster.sh` are needed for NFS configuration as well, which is implemented in `scripts/helper_functions.sh`. I originally wanted to refactor the functions in `install_gluster.sh` so that it can be reusable for the NFS option, but the functions in `install_gluster.sh` are not very modular, but dependent on global variables, so they aren't easily refactor-able, which kind of forced me to re-implement almost same functionalities in `helper_functions.sh`. We should unify these two by rewriting the functions in `install_gluster.sh` so that same functions in `helper_functions.sh` can be reused by both `install_gluster.sh` and `install_moodle.sh` (for NFS setup) as well.

This issue is an example of more refined/concretized work item for #79.