installation: WSGI user in Apache virtual hosts

• BETTER Apache virtual environments are now created with appropriate WSGIDaemonProcess user value, taken from the configuration variable CFG_BIBSCHED_PROCESS_USER, provided it is set. This change makes it easier to run Invenio under non-Apache user identity.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>