Capture Manager should use contexts

[timraymond]

CaptureManager is often responsible for interacting with multiple APIs, including the k8s apiserver. It's apparent from PRs that add new functionality to CaptureManager that there is many context.TODO()s in places where these interactions take place (see: https://github.com/microsoft/retina/pull/295/files#diff-99babd138c7f336f6abec8bd499b3593610bad046a34d47e8d448a7600040c7bR104) . This could be added with a bit of work, binding to the os.Signal channel. While this might ultimately yield the same result, it at least establishes a context heirarchy that will be more easily iterated on in the future.