Use context to cancel in-flight requests

[linki]

Now that client-go supports a context parameter we can actually support it in chaoskube as well.

I've never used context for anything else other than signal handling (like in chaoskube's main loop) and request timeouts. Both could be applied here but for now I'll use it for the former.

Before, if for some reason the call to the Kubernetes API was hanging, hitting Ctrl+C would just not work. Although the top-level context would have its cancel func called, we'd never read the Done channel until the request is completed, e.g. via some timeout.

Now, the top-level cancellable context is propagated down to client-go and Ctrl+C will actually preempt long running requests and the program can terminate immediately.