Make database downgrade works

Fixes bug #854905

Use sqlalchemy reflection whenever possible:
    http://sqlalchemy-migrate.readthedocs.org/en/latest/versioning.html

Work around sqlalchemy-migrate sqlite 'bool column not deletable' issue:
    http://code.google.com/p/sqlalchemy-migrate/issues/detail?id=143
    Add following sql scripts for sqlite:
        002_sqlite_downgrade.sql
        015_sqlite_downgrade.sql
        033_sqlite_downgrade.sql
        050_sqlite_downgrade.sql
        068_sqlite_downgrade.sql

Work around sqlalchemy-migrate sqlite 'table with foreign key column not
deletable' issue:
    http://code.google.com/p/sqlalchemy-migrate/issues/detail?id=94
    Add following sql scripts for sqlite:
        003_sqlite_downgrade.sql
        006_sqlite_downgrade.sql
        007_sqlite_downgrade.sql
        012_sqlite_upgrade.sql
        013_sqlite_downgrade.sql
        020_sqlite_downgrade.sql
        030_sqlite_downgrade.sql
        038_sqlite_downgrade.sql
        042_sqlite_downgrade.sql
        053_sqlite_downgrade.sql
        067_sqlite_downgrade.sql

Work around sqlalchemy-migrate 'migrate drops engine reference' issue:
    http://code.google.com/p/sqlalchemy-migrate/issues/detail?id=72

Add following sql scripts for long primary key to work with utf-8
mysql table:
    072_mysql_upgrade.sql

Add following sql scripts for postgresql:
    002_postgresql_downgrade.sql

Add snake walk test cases for database migration based on glance migration
test.

Change-Id: Ib454ecb4662bbf47736c1b12d9a4f969f180ceb6

nova/db/sqlalchemy/migrate_repo/versions/002_postgresql_downgrade.sql

@@ -0,0 +1,20 @@
+BEGIN;
+
+    DROP TABLE certificates;
+    DROP TABLE consoles;
+    DROP TABLE console_pools;
+    DROP TABLE instance_actions;
+    DROP TABLE iscsi_targets;
+
+    ALTER TABLE auth_tokens ADD COLUMN user_id_backup INTEGER;
+    UPDATE auth_tokens SET user_id_backup = CAST(user_id AS INTEGER);
+    ALTER TABLE auth_tokens DROP COLUMN user_id;
+    ALTER TABLE auth_tokens RENAME COLUMN user_id_backup TO user_id;
+
+    ALTER TABLE instances DROP COLUMN availability_zone;
+    ALTER TABLE instances DROP COLUMN locked;
+    ALTER TABLE networks DROP COLUMN cidr_v6;
+    ALTER TABLE networks DROP COLUMN ra_server;
+    ALTER TABLE services DROP COLUMN availability_zone;
+
+COMMIT;