Stealth Cloud: How not to waste CPU during grid to cloud transitions

UKI-LT2-IC-HEP is a WLCG tier-2 comprising around 4000 job slots and 3.7 PB of storage supporting LHC and non-LHC VOs. When first looking at converting a part of our site's grid infrastructure into a cloud based system in late 2013 we needed to ensure the continued accessibility of all of our resources during a potentially lengthy transition period. To accomplish this we brought together a number of existing middleware and cloud tools. This proved to be a viable long-term solution to maintain resource usage during extended periods of transition.