Implement listing all model information stored in modelmgmtservice

Implement listing all model information stored in modelmgmtservice

- Add an API to list all model information managed in modelmgmtservice
- Add the API testcase to verify whether it works or not
- Modify 'ModelInfo' struct as 'Bucket'
- Tidy some code to follow API and DB perspective
- Remove 'getModelInfoList' route and Update 'getModelInfo' route to
  list all model informations
- Remove unused comment

Issue-ID: AIMLFW-128

Change-Id: I0aded193271327379f4d1ee661eb2c8695acbf64
Signed-off-by: gyuyoung <gyoue200125@gmail.com>