Removes properties

- che.infra.kubernetes.username : can change, requiring reconfiguration
- che.infra.kubernetes.password : can change, requiring reconfiguration
- che.infra.kubernetes.oauth_token : expires

as they complicate setup and all represent suboptimal running scenarios.
Use che serviceaccount instead.

Signed-off-by: Angel Misevski <amisevsk@redhat.com>