toaster: Remove DATABASE_URL being passed around as an environment var

toaster: Remove DATABASE_URL being passed around as an environment var

We don't need to pass the DATABASE_URL around and read it back if we
setup the django framework in the correct way.
We make the default sqlite database path a full path so that the
database isn't being assumed to be in CWD.

Also add some more useful comments on the database settings.

This is preparation work to migrate the build tests and be able to
trigger builds on differently configured databases.

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>