Make container providers opt-in

[timstclair]

The manager should take a list of container handlers to install. The --docker-only flag should be deprecated.

EDIT: Repurposed issue. Was:

We've seen a few issues around the current mechanism that attempts to connect to the rkt service unconditionally (e.g. #1481), and it also produces some log noise.

Is there a better way to detect whether rkt is installed on the host before attempting to connect? Maybe checking for a specific process, or looking for the binary?

[euank]

Checking for the binary won't work if it's not in your path.

Is there a reason not to make rkt integration opt-in and have kubelet instantiate cadvisor with or without rkt support depending on the runtime it's configured with?

[timstclair]

opt-in SGTM. Add a list of handlers to install to the manager interface, and maybe replace the --docker-only flag with a flag that takes a list of handlers (raw, docker, rkt for now).

[amcrn]

any update on this? we're still stuck running v0.23.8 because of it. thanks!