Order pacman-init and reflector-init before cloud-final

http/install-common.sh:
Order pacman-init.service and reflector-init.service before
cloud-final.service, as the latter may install packages using pacman and
will introduce a broken pacman keyring if started simultaneously with
pacman-init.service.
The mirrorlist should be set before cloud-final.service is running, so
that pacman can use it.

.gitlab-ci.yml:
When testing the cloud image using cloud-init:
* Use the `packages` and `runcmd` directives to install packages using
  cloud-init (which is done during `cloud-final.service`).
* Check for packages installed via cloud-init.
* Write a test file to disk and check for its existence.

Fixes #121
5 jobs for issues/121 in 27 minutes and 59 seconds (queued for 3 seconds)