CCO: implement a cost/reliability tradeoff for spot instances

[ilya-kolchinsky]

As of now, the sole objective of CCO is to minimize the overall deployment cost. However, in cases where spot instances are employed, we would also like to make sure our workload is not interrupted too much. Ideally, we would like to combine these two objectives (the cost and the reliability) and provide a way for the user to prioritize between them.
Extend the optimization engine to consider an arbitrary weighted combination of the above two objectives.