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.

The sample config file that ships with osTicket should use the ROOT_DIR
variable to identify the main install path (on the filesystem) of osTicket.