List all tiers

This api offers the interface to list all tiers.
The tiers list will be returned.
---
tags:
  - Tiers
definitions:
  Tiers:
    type: object
    properties:
      creds_name:
          $ref: '#/definitions/Tier'
  Tier:
    type: dict
responses:
  200:
      description: List all tiers
      schema:
          $ref: '#/definitions/Tiers'
      examples:
          "tiers": ""