labs: increase RAM allocation for controller node

The controller node is running most of the services. It slows to a
crawl once all of them are installed.

This patch gives the controller node 1024 MB of RAM (instead of the
current default: 512 MB).

Change-Id: I231fc4e32ed71edf10987a462714bb9d230647d1
Implements: blueprint openstack-training-labs
Merge "labs: increase RAM allocation for controller node"