Don't deploy the docker socket proxy onto every node, but only the necessary ones

[s4ke]

Right now the tecnativa proxy is deployed onto every single node. For small clusters this is probably fine, but we should adapt the logic in the plugin so that it does not deploy the tecnativa proxy onto nodes that are not relevant. Instead it should suffice if we first get the node id that runs the task we want to exec into and then use a placement constraint in the stack to deploy the service only where we need it.