Don't rely on implicit autocommit

Resolve the following RemovedIn20Warning warning:

  The current statement is being autocommitted using implicit
  autocommit, which will be removed in SQLAlchemy 2.0. Use the .begin()
  method of Engine or Connection in order to use an explicit transaction
  for DML and DDL statements.

Change-Id: Ib789cd4d11a3d5dd01fcdb99822025b11bbc234e
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>