Add support of exporting project labels

[freeseacher]

It will be grate if gcppromd could export project labels list.
Let me explain usage:

We are using one project per product and project labels to hold all meta about virtual machines in it.
Adding project level labels will remove duplication for such labels as

• product_name (project_code),
• product_env_type (prod|rc|dev)
  and other common labels for project