Add support for multiple database engines

[OpenStack Infra]

https://review.openstack.org/159266
commit ba407e33d3fedd2d283ba8752ff592fb3e0e2d43
Author: Dheeraj Gupta <email address hidden>
Date: Tue Jan 27 11:55:55 2015 +0000

    Add support for multiple database engines

    This allows for the creation and use of multiple database engines.
    nova-manage database commands will be able to use the proper database
    connection string based on which database they're intended to work with.
    This removes the overloaded use of CONF.database.connection to point at
    the api or main database, and any potential confusion that could result.

    A new config group 'api_database' has been added with the following
    options which are used when creating a sqlalchemy engine:

    connection, sqlite_synchronous, slave_connection, mysql_sql_mode,
    idle_timeout, max_pool_size, max_retries, retry_interval, max_overflow,
    connection_debug, connection_trace, pool_timeout

    DocImpact
    Co-Authored-By: Andrew Laski <email address hidden>
    Change-Id: Ibd197ec431b052e9c4b3fb05bc4f44ee2f3fbdd4
    Partially-Implements: bp cells-v2-mapping

[Gauvain Pocentek (gpocentek)]

The new options are now in the config reference.