frontend.db: Adding afe custom database engine

frontend.db: Adding afe custom database engine

Turns out that we rely on custom code to generate
joins that does some trickery with Django ORM system,
and was using a method calling get_from_clause, that
was present on the Query class on Django 1.1. Now,
the method was moved to a class upper in the hierarchy,
called SQLCompiler, and SQLCompiler is tied to the db
backend being used. After some research, seems like there's
no solution cleaner than implementing one db backend
special for autotest, just to accomodate this function.

So created one db engine that inherits pretty much everything
from the mysql one, except for the compiler classes.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
git-svn-id: svn://test.kernel.org/autotest/trunk@5408 592f7852-d20e-0410-864c-8624ca9c26a4