Execute container commands in parallel

Commands able to use this parallelisation are `stop`, `kill` and `rm`.

We're using a backported function from python 3, to allow us to make
the most of a pool of threads without having to write the low level
code for managing this ourselves.

A default value for number of threads is a low enough number so it
shouldn't cause performance problems but if someone knows the
capability of their system and wants to increase it, they can via
an environment variable DEFAULT_MAX_WORKERS

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

setup.py

@@ -33,6 +33,7 @@ install_requires = [
     'docker-py >= 1.2.3, < 1.3',
     'dockerpty >= 0.3.4, < 0.4',
     'six >= 1.3.0, < 2',
+    'futures >= 3.0.3',
 ]