[nailgun] Add tests to cover database migrations

[Ihor Kalnytskyi]

One more time we have struck against of migration issue:

    https://bugs.launchpad.net/fuel/+bug/1391525

So we MUST cover all migrations with unit tests and we need to have some util stuff to do that.

[Evgeniy L (rustyrobot)]

We should figure out how it should be done.
I think for each version we should have fixtures, like here [1], but it doesn't cover our cases, because this nodes are node deployed as result there is not enough data.
Any ideas?

[1] https://github.com/stackforge/fuel-web/blob/master/nailgun/nailgun/fixtures/sample_environment.json

[Ihor Kalnytskyi (ikalnytskyi)]

We already have some skeleton for doing this - https://github.com/stackforge/fuel-web/blob/7b4350afb2a13eba11c710aa30e6441ec2993fd5/nailgun/nailgun/test/unit/test_migration_fuel_6_1.py#L23-L191

Perhaps it's not optimal, but it works. We should write more tests for fuel_6_1.py migration.

[Ihor Kalnytskyi (ikalnytskyi)]

Mark it fixed since our migrations for 6.1 and 7.0 are covered, and it makes no sense to introduce them for old releases because we don't change them.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/327063

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/327063
Committed: https://git.openstack.org/cgit/openstack/fuel-web/commit/?id=55c095b20b3296de5129bd9a6eb076201f2a6dc4
Submitter: Jenkins
Branch: master

commit 55c095b20b3296de5129bd9a6eb076201f2a6dc4
Author: Nikita Zubkov <email address hidden>
Date: Wed Jun 8 15:31:43 2016 +0300

    Add downgrade migration tests

    Change-Id: Idc79102273d93cd38bc1282aa003ece00ac88d63
    Partial-Bug: #1391553