Commit transaction in migration

Some databases (e.g., DB2) would deadlock on migration 16 because a
lock was held on the domain table for an uncommitted alter and then
SQLAlchemy queried the domain table info in a separate transaction.

The fix is to commit the ALTER statements to release the lock so
that the domain table schema query will not block.

I've run this with sqlite, mysql, and postgresql and they all worked.

Part of fix for bug 1188785

Change-Id: Ic540a6cb09a0c525df7aaea55b64af96f0dd87c7

keystone/common/sql/migrate_repo/versions/016_normalize_domain_ids.py

@@ -347,6 +347,8 @@ def upgrade_user_table_with_col_create(meta, migrate_engine, session):
     session.execute('ALTER TABLE "user" ADD CONSTRAINT user_dom_name_unique '
                     'UNIQUE (domain_id, name);')
 
+    session.commit()
+
 
 def upgrade_project_table_with_col_create(meta, migrate_engine, session):
     # Create the domain_id column.  We want this to be not nullable