Support multiple database connections in the SQLAlchemy storage implementation

[Kiall Mac Innes]

Support multiple database connections in the SQLAlchemy storage implementation, this will allow for connections to additional databases e.g. PowerDNS.

The current use of global _ENGINE and _MAKER variables in session.py prevents this.

At the same time - we should continue to support multiple sessions on a single engine.

[JC Martin (jemartin)]

Since Moniker and the backend might use different engines, it seems that the goal is more to provide a generic SQLAlchemy library to facilitate backend development.
This would allow each implementation to have its own engine, and each engine have multiple sessions.
In addition, the configuration related to SQLAlchemy should provide a kind of name space to allow the configuration of each engine independently

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/17103

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/17103
Committed: http://github.com/stackforge/moniker/commit/f8df098d3059d6a9c58395348eb876394d825fe4
Submitter: Jenkins
Branch: master

commit f8df098d3059d6a9c58395348eb876394d825fe4
Author: JC Martin <email address hidden>
Date: Mon Dec 3 14:04:52 2012 -0800

    Partial fix for bug #1074093

    engine and makers are stored in dictionnary in the
    sqlalchemy/session.py

    Change-Id: Ied2c13e9ed7117730eccc8db4c3a03b54c66d4b9