5864 fixed thread pool limit insufficiency on cascade parallel execute

[ko10ok]

Resolves docker#5864

[pagelypete]

Thanks for your work on this, when running with this branch you get the following error when doing up:

Traceback (most recent call last):
  File "bin/docker-compose", line 3, in <module>
  File "compose/cli/main.py", line 67, in main
  File "compose/cli/main.py", line 126, in perform_command
  File "compose/cli/main.py", line 1070, in up
  File "compose/cli/main.py", line 1066, in up
  File "compose/project.py", line 650, in up
  File "compose/parallel.py", line 138, in parallel_execute
  File "compose/parallel.py", line 242, in producer
TypeError: do() got an unexpected keyword argument 'limit'

Are both this PR and #1 supposed to be merged to fix functionality, or just one of the two?