Add additional config keys for swift

[nightfury1204]

Fixes #4886
This also enables v2 auth for swift.

[briankassouf]

Thanks for the submission! Could you also add these new values to the docs page for the swift backend? You can find it here: https://github.com/hashicorp/vault/blob/master/website/source/docs/configuration/storage/swift.html.md

[nightfury1204]

Updated doc. PTAL

[briankassouf]

Thanks!