[Openstack|volume] Add #get to volumes collection

This fixes reloading Volume objects and makes it consistent with other
collections.

Keeping find_by_id as an alias for compatibility.

lib/fog/openstack/models/volume/volumes.rb

@@ -12,13 +12,14 @@ module Fog
           load(service.list_volumes(detailed).body['volumes'])
         end
 
-        def find_by_id(volume_id)
+        def get(volume_id)
           if volume = service.get_volume_details(volume_id).body['volume']
             new(volume)
           end
         rescue Fog::Volume::OpenStack::NotFound
           nil
         end
+        alias_method :find_by_id, :get
       end
 
     end