Merge remote-tracking branch 'remotes/stsquad/tags/pull-rc3-testing-261119-1' into staging

A few vm-test updates

  - use Ubuntu 18.04 for i386 image
  - python3 for centos and docker
  - install locales for ubuntu

# gpg: Signature made Tue 26 Nov 2019 11:31:26 GMT
# 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-rc3-testing-261119-1:
  tests/vm/ubuntu: update i386 image to 18.04
  tests/vm/ubuntu: include language pack to silence locale warnings
  tests/vm/centos: fix centos build target

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