gbp: rename gbp.conf section
gbp: rename gbp.conf section

Was complaining with:

  gbp:warning: Old style config section [git-buildpackage] found please
  rename to [buildpackage]

Change-Id: I86f8445752bf77fe699fef37d86eae5053c23ef0

wikimedia/integration-zuul,wikimedia/integration-zuul,wikimedia/integration-zuul,wikimedia/integration-zuul