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 I didn't want to load anything in memory as the cloud provider could have better knowledge on how to efficiently parse the config file. I could have used io.Reader but again I felt like I was imposing something more restricting than a "path to a config file".

Maybe going forward we could try to standardize the config file format (but I think it could be a problem for @derekwaynecarr use-case).

Personally I like: http://code.google.com/p/gcfg/

And I will probably use it (gcfg.ReadFileInto) for a cloud provider that I am working on.

If you really feel like we should pass []byte or io.Reader I will update the patch but personally I liked the config file path the best.