Restart fails with column references in rules

[Tim Hinrichs]

When you restart Congress and (i) you have rules that use column references and (ii) you have deleted the datasource driver for the table in which column references appear, Congress fails to restart since it cannot read in the rules.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/167032

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/167032
Committed: https://git.openstack.org/cgit/stackforge/congress/commit/?id=c960d8e822f4f7b248c1d87abee721fae0684f3d
Submitter: Jenkins
Branch: master

commit c960d8e822f4f7b248c1d87abee721fae0684f3d
Author: Tim Hinrichs <email address hidden>
Date: Mon Mar 23 15:05:11 2015 -0700

    Disallow deletion of datasource if dangling references

    Previously, you could delete a datasource even if there
    were policy statements referencing the tables in that
    datasource.

    This change ensures it is impossible to delete a datasource
    if you still have references to that table provided by that
    datasource. This change fixes a bug where Congress
    fails to start because it does not have a schema for the
    rules contained in the database, which can only happen
    if the user has deleted the datasource with rules that
    still reference it.

    Closes-bug: 1435592
    Change-Id: I040f2829fc284b1ddb954b757cea886a498171b4