Add ability to convey application configuration to providers

[kevin-bates]

Kernel providers, for the most part, will likely require configuration to perform their operation. For example, the YarnKernelProvider may need to know the url of the YARN resource manager against which applications (kernels) are submitted. Rather than ask administrators to add this information into the specific kernel specification (however that provider implements that), it would be good to have a central default value capability - where use of the Config class in traitlets makes sense.

As a result, we should add the ability for applications that leverage the kernel provider framework to convey the application's configuration to the providers. The provider would then be responsible for locating its "section" of the configurable classes or the configurable item itself within the set of configurations.