k8s backend domain envvar support

[pl4nty]

To support dynamic environments (eg staging), the k8s backend could support environment variables for at least the domain (eg RCDS_K8S_DOMAIN). A similar feature is implemented in the rCTF backend for its URL and token.

[pl4nty]

I can make a PR this weekend if approved - just want to make sure this feature doesn't clash with any planned changes to the config management architecture (#113).

[ethanwu10]

Unfortunately we don't have the resources right now to implement a lot of the changes that we think rCDS needs, so most of the todo list in #113 is shelved for the time being. I'd be happy to take PRs for this, your proposal looks good to me. I suppose in addition to the domain, the kube context name should also be settable via env var too.

[pl4nty]

No worries. I got sidetracked a bit this weekend, but I'll send through a few PRs now that my prod environment is mostly built. Can't commit to anything major unfortunately.