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 pass an
arbitrary string to the cloudprovider Factory.

Each provider could use such string to receive the relevant parameters
or as a path to a configuration file.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Fine by me if we want to make explicit that it has to be a file path. As I mentioned in the commit message: "Each provider could use such string to receive the relevant parameters or as a path to a configuration file."
I thought that maybe some providers may just need one or two simple (non-sensitive) parameter(s) and they could have squeezed them in the cloud_config string.

Anyway again, I have no problem to force this to be a path to a config file.