Support application credential auth for Openstack swift

[SungJin1212]

Currently, the only way to auth is to use username/password when users use Swift backend. But there is a case where users don't know their OpenStack account but application credentials issued by others (administrators of Openstack account, they don't want to expose Openstack account's username/password). To address this concern, support an application credential auth for OpenStack swift backend so that users can auth in various ways (username/password or application credential).

Which issue(s) this PR fixes: Fixes #

Checklist

• [ ] Tests updated
• [x] Documentation added
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]