Update to virtualenv 1.10.1 (from pypi); fix hpcs load balancer node creation

Updates to venv 1.10.1 (which includes pip 1.4 and comes from pypi rather than relying on git url)
Switches order hpcs load balancer nodes are configured; adds new ones then deletes old ones, to get round a restriction that a load balancer must have at least one node which was preventing e.g. wholesale port number changes.