TMUP-373: Add realpath_root instead of document_root as a flag.

ZendOpcache has problems caching entries based on the symlink.
This stops PHP receiving the symlink and caching php files that change
between deployments.
Merge pull request #20 from inviqa/feature/TMUP-373-zendopcache-fix

TMUP-373: Add realpath_root instead of document_root as a flag.