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>