Expose libclouds' base_url property as an OpenStack configuration.

Apache libcloud has a property called 'ex_force_base_url' which
allows the user to use the specified URL as the API endpoint
instead of using the URL in the endpoint service catalog. This patch
exposes such function to the provider definition as a 'base_url',
and is totally optional. 

libcloud docs: https://libcloud.readthedocs.org/en/latest/compute/drivers/openstack.html#skipping-the-endpoint-selection-using-service-catalog-by-providing-ex-force-base-url-argument