Don't error when trying to start a running container

This changes the behavior of `docker start`. Now, it does not fail when trying to start a running container.
This was already expected but was not the case (see integration/container_test.go l353 and l389).

The idea is to allow `docker start -a <container name>` to work all the time without having to do check if the container exists, if yes attach if no start then attach.

I could have simply updated `start -a` to not error if the container were already started, but as we already expect to be able to start a started container, I changed it for the whole command.