Check for pidMode task in the task definitions of ecs services

[vinnymac]

If the pidMode is set to task, it would be nice if amazon ecs exec checker could warn and point toward the following issue
aws/containers-roadmap#2268

It's very time consuming to search for reasons why exec might not be behaving correctly. The reason that exec isn't working may be because a container happens to not be in the first position when pidMode is set to task. An immense amount of time could be saved if we warned developers ahead of time using a script like this one to educate and avoid headaches.

Currently the script gives you a false sense of security in the scenario above, as it tells you everything is configured correctly while the feature remains broken. Since the cluster is passed to the script, I assume iterating over the list of services, their tasks, and any respective task definitions for a pidMode of task shouldn't be particularly challenging.