Remove required flag from `project-id` and get its value from GCE metadata

[kirecek]

This snippet sets the projectID to value fetched from GCE metadata:

    if *projectID == "" {
        *projectID = getGCEProjectID()
    }

However, this does not work because projectID cannot be empty due to required flag in kingpin config.

This MR removes the required thing and adds a little bit logging around the projectID setting.