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 specify 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.
Merge pull request #12643 from meteorfox/expose-OS-base-url

Expose libclouds' base_url property as an OpenStack configuration.