[OpenNebula] Sunstone frontend eats excessive amounts of memory over time

Over time the frontend allocates significant amounts of memory. I suspect that this is due to the session cache. Either way we should probably use the intended setup:

https://docs.opennebula.org/5.2/deployment/sunstone_setup/suns_advance.html

Given that we don't really want a deployment where we have to restart the front-end every couple of days, I'm giving this a high priority.