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.
Fair enough, although there's nothing to prevent cloudproviders as treating parameter as something other than a config file. Passing an io.ReadCloser seems elegant, actually.