Don't call 'begin()' on existing transaction

Resolve the following RemovedIn20Warning warning:

  Calling .begin() when a transaction is already begun, creating a 'sub'
  transaction, is deprecated and will be removed in 2.0.  See the
  documentation section 'Migrating from the nesting pattern' for
  background on how to migrate from this pattern.

Change-Id: I59e45dfea8fcbf72fc6e34345e510554cbdd138e
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>