Add DNS status check

[lilic]

Sometimes DNS is not configured correctly, mainly on minikube Kubernetes clusters. This checks that the DNS deployment has all the running pods, if not it errors out and suggests to the user to correctly setup DNS.

Fixes https://github.com/weaveworks/launcher/issues/158

Notes:

• I exit rather than just output the error, I noticed in the case of when GKE setup fails you just notify the user but do not abort, not sure which is the correct approach here?
• Was not sure if we should leave a suggestion on how the user should fix this? If so, which of the two options from the issue should we suggest?

[lilic]

Closing in favor of https://github.com/weaveworks/launcher/pull/164