Remove static class loader

This commit remove the static class loader and add a dynamic one.
This is done using a new property available_classes that checks for
classes inherited by Services and generate a list of it.
It also change how tempestconf identify available services using the
list_services api instead of get_catalog api from tempest lib.
This way, tempestconf are no longer generating Services objects for
services that doesn't have a specialized class, but still set the
defaults for true or false if a service is running or not.
The discover method has changed to get the services from the list of
services instead of from the catalog.

Change-Id: Ic9d482378e9c975ed00f6aa7ed5f7aed6956d229
Story: 2002787
Task: 22672

config_tempest/services/image.py

@@ -65,6 +65,10 @@ class ImageService(VersionedService):
     def get_supported_versions(self):
         return ['v1', 'v2']
 
+    @staticmethod
+    def get_service_name():
+        return ['glance']
+
     def get_catalog(self):
         return 'image'