Support specifying kubectl context in kubernetes executor

Problem

The Kubernetes executor contains a footgun: If no host is specified, it will try to guess the kube config.

• If host is specified, use that.
• If the gitlab-runner is running inside kubernetes itself, it will attempt to use that.
• Else, it defaults to the current kubectl context.

This means that if you are running gitlab-runner locally, and have various contexts configured for production access, e.g. via glsh, you run the risk of the local jobs being sent to a production cluster.

Proposal

We should allow the context to be specified explicitly.

cc @rehab