db: Don't rely on branched connections

We were previously calling 'connect()' on the 'connectable' object in
'run_migrations_online', regardless of whether it was an 'Engine' or
'Connection' object. This worked because, as noted in an inline comment,
"when connectable is already a Connection object, calling 'connect()'
gives us a *branched connection*." This is no longer the case. From the
SQLAlchemy docs [1]:

  The Connection object does not support "branching", which was a
  pattern by which a sub "connection" would be used that refers to this
  connection as a parent.

Update our code to reflect this change, using the newly updated example
from the SQLAlchemy cookbook doc [2] as inspiration.

[1] https://docs.sqlalchemy.org/en/14/core/future.html#sqlalchemy.future.Connection
[2] https://alembic.sqlalchemy.org/en/latest/cookbook.html#connection-sharing

Change-Id: I95fc21e1d0993de94a4eb61b2b51ada7ed81044b
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

keystone/common/sql/migrations/env.py

@@ -176,13 +176,21 @@ def run_migrations_online():
             prefix="sqlalchemy.",
             poolclass=pool.NullPool,
         )
+        with connectable.connect() as connection:
+            context.configure(
+                connection=connection,
+                target_metadata=target_metadata,
+                render_as_batch=True,
+                include_name=include_name,
+                include_object=include_object,
+                process_revision_directives=autogen.process_revision_directives,  # noqa: E501
+            )
 
-    # when connectable is already a Connection object, calling connect() gives
-    # us a *branched connection*.
-
-    with connectable.connect() as connection:
+            with context.begin_transaction():
+                context.run_migrations()
+    else:
         context.configure(
-            connection=connection,
+            connection=connectable,
             target_metadata=target_metadata,
             render_as_batch=True,
             include_name=include_name,