Allow reuse of the roles for other directories than doc/build/html with
making doc_build_dir a variable that can be overwritten. This allows to
use the role also for e.g. deploy-guide that lives under
deploy-guide/source and builds under deploy-guide/build.
Change-Id: I61cd84be201d14d0a1d5daba3a45c8797496bc7f
Needed-By: Ifff767a95e6283c54033bababd3dae84bb19adb6