Support passing cloud-config user data to google driver

⚠ Build on top of !42 (merged) ⚠

After merging it please change the target branch to master and proceed with merging this MR

What critical bug this MR is fixing?

Adds support for passing user data in cloud-config format to the created GCP instance. This is required to start the iptables-restore service in Google Container Optimized OS.

How does this change help reduce cost of usage? What scale of cost reduction is it?

In what scenarios is this change usable with GitLab Runner's docker+machine executor?

How to test the MR

1. Login on one of our private runner managers

2. Download the Docker Machine binary build for this MR

   wget https://gitlab-docker-machine-downloads.s3.amazonaws.com/support-google-cloud-config/docker-machine-Linux-x86_64
   chmod +x ./docker-machine-Linux-x86_64

3. Copy the cloud-config configuration file that was prepared by our cookbook:

   sudo cp /etc/gitlab-runner/cloud-config.conf ./

4. Create Docker VM with Docker Machine

   ./docker-machine-Linux-x86_64 \
       create \
       --driver google \
       --google-project gitlab-ci-155816 \
       --google-disk-size 50 \
       --google-machine-type n1-standard-2 \
       --google-username cos \
       --google-use-internal-ip \
       --google-zone us-east1-c \
       --google-machine-image gitlab-ci-155816/global/images/runners-cos-stable-swtich-to-google-cos \
       --google-user-data-file ./cloud-config.conf \
       test-cloud-config-vm

5. Login to the created VM

   ./docker-machine-Linux-x86_64 ssh test-cloud-config-vm

6. Check that iptables-restore service was executed

   sudo journalctl -u iptables-restore.service

7. Check that iptables have our custom rules applied

   sudo iptables -S

   The output can be compared with the definition at https://dev.gitlab.org/cookbooks/packer-runner-machines/-/blob/fc87c887f84e94221b488abd3f902c9a7b0e0227/assets/iptables_rules (GitLab internal access only).

8. Check the user-data metadata entry:

   • by requesting the metadata server:

     curl -H "Metadata-Flavor: Google" http://metadata.google.internal/computeMetadata/v1/instance/attributes/user-data

     It should be update_disabled.

   • by checking the GCP console:

     • go to https://console.cloud.google.com/compute/instances?project=gitlab-ci-155816 and filter the list of VMs with test-cloud-config-vm (or try this link)
     • open the test-cloud-config-vm instance
     • scroll down to the Custom metadata section.

     There should be the user-data entry with the content of https://gitlab.com/gitlab-cookbooks/cookbook-wrapper-gitlab-runner/-/blob/a8ff2098ebcc40166b3a35e2e45b24e40c51cd55/files/default/cloud-config.conf as the value

9. Logout from the VM and remove it

   ./docker-machine-Linux-x86_64 rm test-cloud-config-vm

Related to #14 (closed)