setup_pbuilder: only use ppa on newer releases

setup_pbuilder: only use ppa on newer releases

* only use GCC-7 offered by ppa repo on mimic and newer release.
  we are dropping the support of trusty in mimic and newer releases,
  but we still need to use the old toolchain shipped with trusty to
  build jewel and luminous. so, only use GCC-7 on trusty for building
  mimic and up. as it should be safe to use the new GCC ABI on the
  supported distros, like xenial.
* extract the ppa setup related code into build_utils.sh, so the
  setup_pbuilder scripts can reuse it.

Signed-off-by: Kefu Chai <kchai@redhat.com>