Provide initial set of node labels during swarm join

As of now, setting a node's labels is a task that can be performed by a manager through docker node update after a node has already joined the cluster. However, the following scenarios would benefit from the ability to specify an initial set of labels through something similar to docker swarm join --label key=value:

• A global or replicated service with a constraint of node.label.key!=value might deploy a task in the new node before the operator labels the node with key=value
• The formation of a multi-node cluster where a labelling scheme is known a priori requires an open connection to a manager node just for labelling, assuming secrets or autoaccept are used.
• A global service with a node.role==manager constraint which creates containers that bind-mount the docker socket could utilize an initial set of node labels for node-related metadata.

Start by tracing the docker swarm join path and comparing it with docker node update, focusing on when node labels become available to scheduling. Define the behavior for an initial key=value label set and verify that constrained services cannot schedule before those labels are applied; the issue does not name specific files or tests.