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

[hosungsmsft]

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.