Remove use of Session.begin.subtransactions flag

Resolve the following RemovedIn20Warning warning:

  The Session.begin.subtransactions flag is deprecated and will be
  removed in SQLAlchemy version 2.0.  See the documentation at
  session_subtransactions for background on a compatible alternative
  pattern.

Change-Id: Ib2537bae77861ee60d17a48a72c57b88e043553e
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>