Let a flavor plugin manage configuration across groups

[dgageot]

A nice feature for docker swarm would be to create a Swarm without telling the number of managers and workers but only the number of nodes. The two kinds of instances would still be managed by two scaling groups but a swarm flavor plugin could dynamically adjust the size of each group depending on the total size of the swarm. For eg.:

• 1 node -> 1 manager
• 2 nodes -> 1 manager + 1 worker
• 4 nodes -> 3 managers + 1 worker
• 100 nodes -> 3 managers + 97 workers

To achieve that, the flavor plugin needs to manage configuration across groups.