db: Final cleanups

Some things that were missed in previous patches and are thrown together
here:

- Add alembic as an explicit dependency (we were getting it transitively
  from oslo.db). We also bump the sqlalchemy dependency to a 1.4.x
  release, which is the minimum supported by our chosen version of
  alembic (more on this below)
- Remove tooling related to the old migrations
- Fix the tox whitelisting of the flaky MySQL tests

On the SQLAlchemy front, we opt for 1.4.13. Technically alembic should
support anything from 1.4.0, however, with SQLAlchemy >= 1.4.0, < 1.4.13
we see errors like the following in some tests:

  sqlalchemy.exc.InvalidRequestError: Entity namespace for
  "count(instance_mappings.id)" has no property "queued_for_delete"

There's nothing specific about this in the release notes for 1.4.13 [1]
but it definitely fixes things.

[1] https://docs.sqlalchemy.org/en/14/changelog/changelog_14.html#change-1.4.13

Change-Id: I4c8eb13f11aa7471c26a5ba326319aef245c9836
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tox.ini

@@ -32,11 +32,7 @@ passenv =
 # there is also secret magic in subunit-trace which lets you run in a fail only
 # mode. To do this define the TRACE_FAILONLY environmental variable.
 commands =
-# NOTE(gibi): The group-regex runs the matching tests in the same executor.
-# These tests runs against a real mysql instance and in an IO deprived CI VM they tend to time out.
-# See bug https://launchpad.net/bugs/1823251 for details.
-# By running them in the same executor we can spread the IO load of these tests in time.
-  stestr --group-regex=nova\.tests\.unit\.db\.test_migrations\.TestNovaMigrationsMySQL run {posargs}
+  stestr run {posargs}
   env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
   stestr slowest
 
@@ -96,13 +92,7 @@ deps =
   {[testenv]deps}
   openstack-placement>=1.0.0
 commands =
-# NOTE(gibi): The group-regex runs the matching tests in the same executor.
-# These tests runs against a real db instance and in an IO deprived CI VM they tend to time out.
-# See bug https://launchpad.net/bugs/1823251 for details.
-# By running them in the same executor we can spread the IO load of these tests in time.
-# NOTE(gibi): I was not able to group only the mysql tests this way as regex
-# TestNovaAPIMigrations.*MySQL does not do what I expect
-  stestr --group-regex=nova\.tests\.functional\.db\.api\.test_migrations\.TestNovaAPIMigrations --test-path=./nova/tests/functional run {posargs}
+  stestr --test-path=./nova/tests/functional run {posargs}
   stestr slowest
 
 [testenv:functional-py36]