Allowing to restart docker nodes during simulation

[lhoff94]

This PR adds a command to the DockerNode allows restarting of a container during simulation e.g. by a workflow.

Using this function in a workflow enables the following use cases:

• Testing the behavior of other components if a container restarts
• Testing the restarting behavior of the component that restarts if other nodes are up and running
• Changing the image of the container during simulation e.g. to a newer version or a build with a different configuration
• Changing the parameters of the container e.g. connected volumes, exposed ports, environment variables during simulation

I have to admit lines 249-253 are a bit hacky. They are a workaround for a strange behavior where I couldn't get to the root cause.
If a DockerNode is stopped by calling .stop_docker_container() the veth pair that belongs to it is deleted as well (as far as I can tell by the operating system since there is no call to interface.remove_veth_pair()). The strange thing is that it only happens if the DockerNode is the last one defined in the Scenario. If it is not the last one, the veth_pair survives but is still connected to the namespace of the old container and therefore needs to be deleted and recreated as well.
I tried to find the cause for this behavior but couldn't identify if it's something the kernel or the docker daemon does. However, the solution that is implemented in this PR worked for me for a couple of months during which is used this feature extensively.

[lpirl]

LGTM apart from some nit picks.

Sure we cannot come up with a cleaner solution for 249-253? @arneboockmeyer, can the deletion of the veth pair be related to some kind of automatic clean up?

[lhoff94]

I hacked up an example scenario based on the existing "basic_example.py" to replicate the behaviour that I described here:
https://gist.github.com/lhoff94/ceb72b2b16e2f82359d487b8b204a5cb
The "restart_docker_container" method is embedded into the scenario so that it works with the latest Marvis release (without the commit from this PR).
I also added the mavis log with the exception as a second file to the gist. As you can see, the first container restarts as expected but the second doesn't because the veth can't be deleted because it's not there anymore.

[lpirl]

@arneboockmeyer will have a look :)