ROOT_DIR should be used to identify filesystem paths

Historically, ROOT_PATH and ROOT_DIR contained the same value; however,
ROOT_PATH now points to the URL path where osTicket is installed, whereas
ROOT_DIR points to the file system location where osTicket is installed.
Merge pull request #698 from greezybacon/issue/root-dir

ROOT_DIR should be used to identify filesystem paths

Reviewed-By: Peter Rotich <peter@osticket.com>