Use namespace.Exec() and namespace.Init() to execute processes in CT

This series is the second step to support the new API. Now an init process is actually executed in a new container. This series doesn't change code about creating containers, it only reworks code according with new API.

We are not going to save backward compatibility with old API, aren't we?
Interesting, we never had the static issue in cAdvisor. I think it was because we were already asking for cgo code to be statically linked. Maybe I'm just used to glog's configuration since it is identical to what we use in Google.