Allow environment variable in ports defining section

[lucasvc]

In fig.yml you can pass some environment variables to docker hosts, using environment section. But in ports section you can't reuse those variables, so a replace should be done before executing fig file. Following first example, would be nice to do:

web:
  build: .
  command: python app.py
  links:
   - db
  environment:
   - OUTPUT_PORT
  ports:
   - "$OUTPUT_PORT:8000"
db:
  image: postgres

So executor and container know each other the port value, without having replace any content.

[nubs]

This looks related to #564.

[dnephin]

Closing, discussion can continue in #564