[openstack|image] Retrieve glance version if not set

Some environments (as a default devstack env) don't set the version for glance endpoints in service catalog, so if you try to use Fog::Image, it'll fail with a 300 (Multiple Choices) error. 

Instead of failing, if glance endpoint doesn't have version in path, fetch glance for current glance version. It uses same pattern as [openstack network](https://github.com/fog/fog/blob/master/lib/fog/openstack/network.rb#L215)