Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-next-100919-2' into...

Testing fixes:

  - podman cleanups
  - docker.py python3 fixes (encode)
  - DEF_TARGET_LIST applied to cross build images
  - move a bunch to Buster based images
  - enable Travis caching
  - more common objs for faster builds
  - stable URLs for acceptance tests
  - additional travis dependencies
  - work around ppc64abi32 linux-test breakage [v2]

# gpg: Signature made Tue 10 Sep 2019 14:16:00 BST
# gpg:                using RSA key 6685AE99E75167BCAFC8DF35FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>" [full]
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-testing-next-100919-2: (46 commits)
  tests/tcg: fix typo when calling clean-tcg
  travis.yml: Install libcap-dev for testing virito-9p
  Fedora images: use URLs from stable "archives.fedoraproject.org"
  .travis.yml: Cache Linux/Clang jobs together
  .travis.yml: Cache Linux/GCC 'non-debug profile' jobs together
  .travis.yml: Cache Linux/GCC 'debug profile' jobs together
  .travis.yml: Document how the build matrix use caches
  .travis.yml: Enable ccache on OSX
  .travis.yml: Improve ccache use
  .travis.yml: Cache Avocado cache
  .travis.yml: Cache Python PIP packages
  .travis.yml: Increase cache timeout from 3min to 20min
  .travis.yml: Enable multiple caching features
  configure: check if --no-pie is supported first
  tests/docker: --disable-libssh on ubuntu1804 builds
  tests/docker: add more images to PARTIAL_IMAGES when not on x86_64
  tests/docker: use --arch-only for installing deps
  tests/docker: add debian-amd64-cross for non-x86 hosts
  tests/docker: avoid $SHELL invoke bash directly
  tests/docker: add debian9-mxe to DEBIAN_PARTIAL_IMAGES
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>