Command for listing and removing credentials stored by Renku

[mohammad-alisafaee]

As a Renku user I want to be able to see all saved credentials by Renku so that I can delete them.

Acceptance Criteria

• A credentials ls command that list available sources of credentials not their values

  EXTERNAL SOURCE                   
  ----------------------------------
  s3://server/path/to/bucket      
  dev.renku.ch  
  s3://server/path/to/another/bucket
  

• A credentials rm <name>/--all command that allows deleting credentials for a specific source or for all