Support Swarm templating for driver-opt

[mark-church]

docker/cli#62 introduced the capability to pass in driver-opt to services so that network driver specific options can be sent in per service. Swarm templating on this parameter would be very useful for a variety of scenarios that need to send task or service-specific information to a network driver.

A similar request was also requested by moby/moby#33408.

@stevvooe has offered to help with guidance if there are any contributors that want to take this on.

[benturner]

@stevvooe @mark-church, I'm interested in tackling this, can you guys point me in the right direction?

[stevvooe]

@benturner Template expansion code is in https://github.com/docker/swarmkit/blob/master/template/expand.go#L16. Right now, we do it for the mounts fields and the env field. Just need to have this cover the networking fields.