Support for additional control over assumed roles

[brandond]

• IAM_EXTERNAL_ID variable; if found value will be populated into ExternalId parameter when making AssumeRole call.
• ROLE_SESSION_KEY variable; if found will use value to look up key from Docker container labels or environment variable to set RoleSessionName when making AssumeRole call.
• Reduce number of calls to Docker API when retrieving credentials.
• Bump WSGI versions to add support for gunicorn --capture-output option
• Set up logging when not run in debug mode

[brandond]

@ryan-lane For some reason your commit tagging 1.3.2 isn't in master, so that got rolled up in here as well since I branched on the 1.3.2 tag. If you want to push to master I can rebase.

[ryan-lane]

I have some comments to be addressed, but overall good PR. The code is getting complicated enough that it's probably a good idea for us to start adding some tests...

[ryan-lane]

Thanks for the contribution btw :)

[brandond]

OK @ryan-lane I think I got everything you asked for. I also fixed the mock routes, since I had forgotten to look at them.

[brandond]

@ryan-lane anything else you need here?

[ryan-lane]

Sorry about the delay. I need to do some manual testing with this.

[brandond]

If it helps any, I've been running it on our GitLab CI box for the last week or so and it's been working great. Probably smaller scale than y'all are running it at though ;)

[ryan-lane]

I'm really sorry for the long wait on this. I'm going to test this properly over the weekend and I think based on a review of the code that it'll be mergeable without change. Thanks again for the code!

[ryan-lane]

Thanks for the contribution!

[ryan-lane]

released in tag 1.5.0