Allow configurable regexp to identify containerd cgroups

We are exploring using cAdvisor for a customized deployment of containerd. In our deployment, we create container names that do not match the `containerdCgroupRegexp` regexp specified in [containerd/factory.go]. As a result, cAdvisor passes on even attempting to [handle] the cgroup.

It's not trivial for us to change the way we name containers.

Would it be reasonable to accept the `containerdCgroupRegexp` as a command-line parameter to allow a more general naming convention of containerd containers?

[containerd/factory.go]: https://github.com/google/cadvisor/blob/5adb1c3bb38b4c5d50b31f39faf3214a44ae479b/container/containerd/factory.go#L44
[handle]: https://github.com/google/cadvisor/blob/5adb1c3bb38b4c5d50b31f39faf3214a44ae479b/container/factory.go#L272