types/swarm: make mounts read write by default
types/swarm: make mounts read write by default

To ensure that the zero-value reflects the defaults for mounts, change
the `Writable` to `ReadOnly`. Adds a breaking api change to address a
point of confusion.

If this PR is not merged by the 1.12 release, this can never be merged,
so just close and move on.

Related to docker/docker#24053.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

jwhonce/engine-api,vdemeester/engine-api,docker/engine-api,thaJeztah/engine-api