Add "-a, --all" option for "backend.ai ps" command

[rjwharry]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like It will be much comfortable if we could list sessions regardless of their status by adding "-a, --all" option for "backend.ai ps" command.

Describe alternatives you've considered

Additional context In docker, we use "docker ps -a" when we want to list all containers including dead one. Based on here, I want to add "-a, --all" option for "backend.ai ps"