expose Adaptive keyword arguments in adapt method

[jhamman]

Our current adapt method does not allow for any keyword arguments. It would be nice if we could pass in arguments like minimum and maximum to the Adaptive constructor with a usage like:

cluster = MyQueueCluster(...)
cluster.adapt(minimum=1, maximum=20)

[mrocklin]

This would be resolved by subclassing from the new distributed.deploy.Cluster superclass.