Show pod name for each deploy replica

What does this MR do?

Populate Deployment objects with current pods running. It associates each pod with deployment
by environment and track(stable or canary).

We do this to show pod names and status on deploy boards.

closes #5242 (closed)

Things to pay attention:

A lot of code has been removed from ee/lib/gitlab/kubernetes/deployment.rb we should make sure not to brake anything.

Suggestions

If this works out we can show some other properties for each pod, like failing reasons, last health check etc.