Merge remote-tracking branch 'remotes/philmd-gitlab/tags/acceptance-testing-20200812...

Acceptance tests patches

- Use stable URLs for the Debian and Ubuntu installer
  (Ubuntu has been updated last Wednesday, August 5, 2020).

CI jobs results:
. https://cirrus-ci.com/build/6385815351721984
. https://gitlab.com/philmd/qemu/-/pipelines/177054604

# gpg: Signature made Wed 12 Aug 2020 14:17:09 BST
# gpg:                using RSA key FAABE75E12917221DCFD6BB2E3E32C2CDEADC0DE
# gpg: Good signature from "Philippe Mathieu-Daudé (F4BUG) <f4bug@amsat.org>" [full]
# Primary key fingerprint: FAAB E75E 1291 7221 DCFD  6BB2 E3E3 2C2C DEAD C0DE

* remotes/philmd-gitlab/tags/acceptance-testing-20200812:
  acceptance: use stable URLs for the Debian and Ubuntu installer
  tests/acceptance/boot_linux: Extract common URL from xlnx-versal test

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