engine: Introducing "cache" for ActionGroupDAO

The following patch is introducing a simle cache for ActionGroupDAO.
The following Dao only perfrom select operation from DB and never update operation.
The following query is running during each engine bll comand, so introducing a cache will decrease
dramaticly quries to DB.
In general I would prefer to use some third-party cache solution which can be integrated inside dao,
but because of it can some time, I just wrote something simple that should be good for now

Change-Id: I292ed3cba6075064e13ba8cee73c47a018fa568a
Signed-off-by: Michael Kublin <mkublin@redhat.com>