- Split out interface detection into intf-exists? function.
- Use 'intf-exists?' to check for 'eth0' (docker host bridge interface)
  and store result in ctx :docker-eth0? property.
- Some minor docs/formating changes.

The function that runs a docker-compose commmand becomes dc_run and
that's all it does (not wait loop). The main test running function that
includes the retry functionality is split out into a non-compose
specific function called do_test. If the WITH_DC environment variable is
set then it uses dc_run to run a command, otherwise it invokes the
arguments in a regular shell call.

The dc_wait and dc_test function become trivial wrappers around do_test.

Allow the minimum ping delay over the netem link to be anywhere from
80ms - 85ms instead of 80ms.

- Add new 'forward' link property. This property is an array of strings
  of the form `conlink_port:container_port/proto` similar to docker port
  publishing.
- Add 'link-forward.sh' script that will add (or delete) the iptables
  and route rules to accomplish the port forwarding from one interface
  to an IP on another interface with port remappping (NAT/masquerade).
- Add a test10 example that shows a single forward definition forwarding
  to two container replicas and shows two forward defintions both
  forwarding to a single container service/port.