container release: r322167

• clang major version release 6.0.0 -> 7.0.0 (revision r322167)
• go minor version release: 1.9.2 -> 1.9.4
• update the sha256 of python-runtime image to include the latest
  changes.
• remove the unnecessary BUILD files from pkg_tar rules.
• also we add container-strucuture-test for the local build
  https://github.com/GoogleCloudPlatform/container-structure-test
• debian8-clang-fully-loaded sha256 digest when build with container-builder:
  b2d946c1ddc20af250fe85cf98bd648ac5519131659f7c36e64184b433175a33