Add as_dict and from_db_model in Base API

Added Base API methods 'as_dict','from_db_model' and 'as_dict_from_keys'
available in ceilometer (http://goo.gl/KwoqVj)
This will be useful for the upcoming patchsets which connect api
models to sqlalchemy models.

Change-Id: I7a52b26671f820a1a19e8b041fada973fd16987d
Merge "Add as_dict and from_db_model in Base API"