Create class "Connection" for sqlalchemy.api

We would like to use more OOP in more parts of rally.
For db API : we would like to have abstract class with
abstract methods instead of a bunch of methods.

This is frist step to achieve the OOP for module db api.

Partially implements: blueprint use-oop-for-db-api

Change-Id: Ie39069dbb30136ff96d36b73d893b557babf19dd
Merge "Create class "Connection" for sqlalchemy.api"