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.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Merge pull request #1160 from simon3z/master

Support configurations for cloudproviders