[#186] Update model definition

[andylowry]

Work on this issue is being done while consulting PR #182, branch task/ZEN-4322b. Unfortuantely, that PR was opened just prior to a major overhaul of the validation framework, that was being done simultaneously. As a result, the PR diffs are nearly useless.

What I'm doing instead is using github to view a diff of that PR branch agaisnt commit 6e2cd8e7d29319dc1f10e35a7f63154fb0e44243, which is the commit just prior to the renovation of validation. I'll use that much more manageable diff to guide the changes in this branch.

[andylowry]

@tfesenko I'll answer questions that I find in your branch for PR #182 here. Commenting on the diff I'm using for this work is not possible, since it's not a PR diff.

• You asked why there's a warning when an Operation summary is longer than 120 chars. That's in the 3.0.0-rc0 version of the spec, which is what was current when I created the validations. I'll remove the validation.
• You asked why the OAuthFlow fields are all, "always" required. The refreshUrl should not have been. The others are all marked in the spec as required, and were required in the 3.0.0-rc0 version that I originally worked from. If it's wrong for these to be required (note that the individual OAuthFlow objecs that can appear in a SecurityScheme are all optional), then the spec will need to be corrected. I'll fix the refreshUrl validation now.