Store Kubernetes credentials in GitLab

Following our discussion about console terminal within GitLab we came across posting the Kubernetes credentials: https://gitlab.com/gitlab-org/gitlab-ce/issues/22864#note_16404398

This issue is to discuss how it should look like, where and what should be defined.

It's good to know that Kubernetes has multiple authentication schemes: http://kubernetes.io/docs/admin/authentication/. We should probably consider supporting at start only some of them.

For some time I thought about introducing separate Credentials cog option, where we would be able to add any external credentials: Kubernetes, SSH private keys, Private Docker Registry or any other.

The goal is to:

1. use internally to connect to for example Kubernetes Pods,
2. use internally to implement health checks,
3. deliver them to GitLab Runner to easily put them in build environment.

Other way to achieve that is extending Services with Kubernetes and allowing the post all credentials there.