Allow programmatically configuring a cloud instance

[lukehoban]

There are many config setting to control details of the cloud-aws implmentation. Currently, these must be provided via ambient external config. It would be nice to programmatically configure a cloud instance instead. For example, to pass a network constructed in AWS as the network configuration.

This is fundamentally very similar to first class providers. We should see if we can use a similar solution to allow creating a new cloud.Provider whose args are config and which can then (optionally) parameterize resources in this package.