Pass variables from deployment project services to CI runner

What does this MR do?

This commit introduces the concept of deployment variables - variables that are collected from deployment services and passed to CI runner during a deployment build. Deployment services specify the variables by overriding "predefined_variables" method.

This commit also configures variables for KubernetesService

Why was this MR needed?

We need these values for https://gitlab.com/gitlab-org/gitlab-ce/issues/23580

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Refers to https://gitlab.com/gitlab-org/gitlab-ce/issues/23580