openapi-spec: support the upcoming 3.0 flavour of OpenAPI Spec

[bajtos]

This is a follow-up issue for #177, it may be a duplicate of #753

[bajtos]

Quoting from https://www.openapis.org/blog/2017/05/25/the-open-api-initiative-is-sending-you-a-save-the-date-card:

At its meeting on Friday, 19 May, the Technical Developer Community (TDC) agreed to target the below timeline for release:

• 4 weeks from 22 May through 16 June: Resolve remaining pull requests
• 2 weeks from 19 June through 30 June: Comment period
• 2 weeks from 3 July through 14 July: Resolve remaining comments and issues
• 1 week from 17 July through 21 July: TENTATIVE release window

[dhmlau]

Talked to @jannyHou , seems like this could be a duplicate of #753, since we're gonna support openapi3.
Closing as dup.
@kjdelisle @raymondfeng , if you don't agree, please reopen. Thanks.

[bajtos]

According to #696, migration to OpenAPI Spec v3 is out of scope of the MVP release. Let's keep this issue open until we actually have v3 support implemented.

[dhmlau]

Closing as it's been resolved.