Add Daemon restart test along with Daemon utils

This PR allows writing tests that need a daemon restart. It uses hack/dind.
There is also a very useful test case that can also serve as an example on how to use it.

Ping @unclejack @tianon @vieux
Three levels of dockerizing are cool :) But can bring some complicated errors. Why we just not run another daemon with another socket, --graph and --pidfile in same container?