[ECS] Need Host environment variable resolution to pass some information to a container

Before moving to ECS, I need to be able to make sure that I can run consul in that environment. To run consul, I need to pass some Host environment variables as environment variables to the container. I would normally do this as `-e HOST=$HOST` etc. However, it seems that that if I define a variable with `$HOST` in a task definition, this is treated as a static string.

Related, I also typically would set docker command line options on the run command for several features I need. I may be able to get around some of this by overriding options in the docker daemon itself, but as far as passing host information I am not aware of any workaround.