cluster: Refuse swarm spec not named "default"
cluster: Refuse swarm spec not named "default"

If, using the API, a user submits an init request with a spec that has a
name other than "default", the engine will rename the "default" cluster
object. Some parts of swarmkit depend on having a cluster object named
"default". Reject any specs that use other names.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

stainboy/docker,medallia/docker,medallia/docker,medallia/docker,stainboy/docker,medallia/docker,stainboy/docker