Support object_storage_url override #341

Generally, the object_storage_url is built from OS_AUTH_URL, OS_PROJECT_ID and other similar variable. In php-opencloud/openstack it's retrieved from the JSON payload obtained from POST /v3/auth/tokens (in token.catalog.endpoints[type=="object-store"][interface == "xx"].url)

Still, the Python Swift client allows for another alternative: OS_STORAGE_URL

This allows to use a distinct storage-url unrelated from said variable.

This is useful (= actually necessary) in the context of application credentials, where an application credential is generated within a given project, but used to access an object container in another project (whose ACLs have been configured). In such a case we can't rely on the URL generation logic.

This is need to give a use for cross-projects #336
Let's remind that some Openstack hosting solution do not offer multiple users, but only multiple projects, making cross-project application credentials the only secure way to grant access to an object container which implies supporting the OS_STORAGE_URL override.

In a more generic way, it means giving the ability to override OpenStack\Identity\v3\Models\Service::getUrl() from the configuration (this class as not access to)