Support configurations for cloudproviders

Cloud providers may need specific configurations to run properly (e.g. authentication parameters, uri, etc.).

This patch adds the simplest implementation for passing configurations to cloudproviders: a new apiserver -cloud_config flag to specify the path to an arbitrary configuration file.
@lavalamp actually now that I think of it, I don't think that io.ReadCloser is going to satisfy @derekwaynecarr use case. What do you think?

@derekwaynecarr  you'd need to have a configuration file to define where is the file to watch. Would that be ok with you?