This commit adds all necessary bits to be able to execute runtime tests
for mender-convert'ed QEMU x86_64 image. Namely:
* Install SSH key as an authorized one on the device using an overlay,
and pass the private one to be use by the test framework.
* Create a QEMU launcher wrapper, similar to the one in meta-mender, for
the test framework to use in order to start/stop the virtual device.
* Exclude the test_update tests for all configurations but Ubuntu QEMU
one using a pytest filter 'not test_upate'.
* Copy from meta-mender the keys required for test_signed_updates.
* Install mender-artifact in CI to be used in tests.
Changelog: None
Signed-off-by: Lluis Campos <lluis.campos@northern.tech>