if exposing docker api locally use a unique port for each box in the cluster

Without this change vagrant will not let you `suspend` and then `resume` a multi-box cluster that is forwarding the docker port. You will get the following error message:

    Vagrant cannot forward the specified ports on this VM, since they
    would collide with some other application that is already listening
    on these ports. The forwarded port to 4243 is already in use
    on the host machine.

    To fix this, modify your current projects Vagrantfile to use another
    port. Example, where '1234' would be replaced by a unique host port:

      config.vm.network :forwarded_port, guest: 4243, host: 1234

    Sometimes, Vagrant will attempt to auto-correct this for you. In this
    case, Vagrant was unable to. This is usually because the guest machine
    is in a state which doesn't allow modifying port forwarding.