installation: Nginx front-end server
- Introduces Nginx front-end web server to web node provisioning scripts to run the demo site installation on port 80, analogously to Docker-based demo site installations.
- Fixes the order of demo site population instructions to suit the strict JSON Schema validation.
- Introduces optional Vagrant cache for deb/rpm packages to efficiently reuse system dependencies among virtual machines.
Signed-off-by: Tibor Simko <tibor.simko@cern.ch>