Add native_attr to OPTIONS of Tower Credentials

[jameswnl]

As a proposed to solution to #14900

Part of bug https://bugzilla.redhat.com/show_bug.cgi?id=1444398

@miq-bot add_labels wip, bug, providers/ansible_tower

[miq-bot]

Checked commit jameswnl@e904d78 with ruby 2.2.6, rubocop 0.47.1, and haml-lint 0.20.0
4 files checked, 0 offenses detected
Everything looks fine. 🍪

[jameswnl]

@mzazrivec will this be helpful?

[mzazrivec]

The problem with this approach is that it introduces inconsistency between what you
get from our rest api when you query credential OPTIONS and what you actually get
when you GET the actual credential resource.

This makes me wonder why we're even telling the world (via api) what info you can
get, when in truth you won't get it. What's the point really?

The situation with the credentials api is already bad as it is (since there's already inconsistency
between what you GET and what you need to POST for it to work) and this just makes
the bad situation worse.

We cannot reasonably expect the api consumers (angular UI or anything else) to understand
semantics of this native_attr (btw. you have typo in your PR: navtive_attr) and create
some obscure if statements for this to make it work.

So no from my end.