Define variable for public_html path in defaults.sh

We moved away from any hardcoded paths in Shell bin scripts,
instead defining them as environment variable in defaults.sh.

Add variable for ./public_html and use it throughout.

This also paves the way to use that path more,
eg to handle the symlinks in the build scripts.

Change-Id: I040fd43f90c0bb14bd07383a979771b464871e8a