Traverse to bottom of graph

[Ton Ngo]

In hot_resource.py, the function bottom_of_chain(self) is used to traverse to the bottom of the graph to find the hosting server. This traversal is done on the HOT graph, which is logically correct, but probably not a good practice. It would be better to traverse the TOSCA graph instead and compute from the relationship requirement. Since this is a common case, it can also be a util function.