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 (closed)