Support passing custom metadata to Google instance

What critical bug this MR is fixing?

Adds support for passing custom metadata to created GCP instance. This is required to disable automatic updates of the 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-custom-metadata/docker-machine-Linux-x86_64
   chmod +x ./docker-machine-Linux-x86_64

3. 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-metadata=cos-update-strategy=update_disabled \
       test-metadata-vm

4. Login to the created VM

   ./docker-machine-Linux-x86_64 ssh test-metadata-vm

5. Check that update-engine service is stopped

   systemctl status update-engine
   systemctl is-active update-engine

6. Check the cos-update-strategy metadata entry:

   • by requesting the metadata server:

     curl -H "Metadata-Flavor: Google" http://metadata.google.internal/computeMetadata/v1/instance/attributes/cos-update-strategy

     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-metadata-vm (or try this link)
     • open the test-metadata-vm instance
     • scroll down to the Custom metadata section.

     There should be the cos-update-strategy entry with update_disabled value.

7. Logout from the VM and remove it

   ./docker-machine-Linux-x86_64 rm test-metadata-vm

Part of #14 (closed)