Allow services with published ports to be scaled to 1

[jamshid]

How about allowing services with a published port (eg, "80:80") to be "scaled" to "1" or "0". Report the below error only when scaling >=2. Sorry I know this is a nit, but seems slightly better behavior and would simplify some scripts.

$ docker-compose scale myserver=1
Service "myserver" cannot be scaled because it specifies a port on the host. If multiple containers for this service were created, the port would clash.

Remove the ":" from the port definition in docker-compose.yml so Docker can choose a random port for each container.

[dnephin]

This was fixed by #1466, which I believe made it into the 1.3 release