Remove the 'Session.autocommit' parameter

Resolve the following RemovedIn20Warning warning:

  The Session.autocommit parameter is deprecated and will be removed in
  SQLAlchemy version 2.0.  The Session now features "autobegin" behavior
  such that the Session.begin() method may be called if a transaction
  has not yet been started yet. See the section session_explicit_begin
  for background.

Change-Id: I7867cdcea115b13f2e45e0674bb9ef2ad138aae9
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>