feat(api): add swagger support at the root of the API

[helgi]

Content Type for it is application/openapi+json or ?format=openapi

Closes #811

Very much so work in progress still

A few off the cuff considerations:

• Auth the endpoint? (use API key). Hooks get hidden from view when using normal tokens, which defeats the point
• throttle access to that endpoint
• save swagger on each release? add into the docs site somehow potentially

[deis-bot]

@kmala, @bacongobbler and @mboersma are potential reviewers of this pull request based on my analysis of git blame information. Thanks @helgi!

[codecov-io]

Current coverage is 86.96% (diff: 80.00%)

Merging #1068 into master will decrease coverage by 0.80%

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- Partials        152        160     +8   

Powered by Codecov. Last update 9f6cb94...9bc9689

[helgi]

When encode/django-rest-framework#4525 (DRF 3.5) is out then I'll revisit most of this

[mboersma]

We are also considering using https://github.com/kubernetes-incubator/client-python since it seems to be swagger-generated and may come close to matching our hand-crafted, artisanal k8s client functionality.

[mboersma]

I'm closing this PR since it's old and rusty, but @helgi please re-open if this code gets resurrected. Thanks again!