db: Enable auto-generation of API DB migrations

Change I18846a5c7557db45bb63b97c7e8be5c4367e4547 enabled auto-generation
of migrations for the main database. Let's now extend this to the API
database using the same formula. While we're here, we also enable
"batch" migrations for SQLite [1] by default, which allow us to work
around SQLite's inability to support the ALTER statement for all but a
limited set of cases. As noted in the documentation [2], this will have
no impact on other backends where "we'd see the usual 'ALTER' statements
done as though there were no batch directive".

[1] https://stackoverflow.com/a/31140916/613428
[2] https://alembic.sqlalchemy.org/en/latest/batch.html

Change-Id: I51c3a53286a0eced4bf57ad4fc13ac5f3616f7eb
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

.gitignore

@@ -49,5 +49,9 @@ tools/conf/nova.conf*
 doc/source/_static/nova.conf.sample
 doc/source/_static/nova.policy.yaml.sample
 
- # Files created by releasenotes build
+# Files created by releasenotes build
 releasenotes/build
+
+# Files created by alembic
+/nova.db
+/nova_api.db