Add support for discovering Kubernetes targets.

[manugarg]

Based on the interactions with the Cloudprober users so far (over Github issues and in person), it's quite clear that cloudprober is used quite a lot within Kubernetes environment to monitor Kubernetes resources -- pods, services, etc. Adding auto-discovery support for Kubernetes resources will greatly help such setups. For example, we could auto-discover all pods having a particular label.

We should extend Cloudprober's RDS server to support Kubernetes resources. In RDS terms, "kubernetes" (or "k8s") will be the provider and supported resource types will be "pods", "services", "endpoints", etc.

[manugarg]

Added framework support and pods as the first implementation in 34b261c.

[manugarg]

Added support for endpoints as well: 808b8f9.

[manugarg]

And, d832a46 adds support for services. I think all we need to do now is to write some documentation on how to use resource discovery in Kubernetes.

[manugarg]

Closing this now. This feature is getting released in 0.10.6. We can do enhancements and other improvements through a separate issue.