Customizable formats for "backend.ai ps" CLI command

[gofeel]

Add some formatting options for ps - It's too long.

suggestions
--full
--format

etc

[achimnol]

We now have --detail as a separate option to the ps command.

[achimnol]

Let's generalize the formatting using --format option, allowing specification of columns.
e.g.,

• ps -f id,created_at,status,status_info -> uses tabulate's default tablefmt.
• ps -f id,created_at,status,status_info --plain -> uses tabulate's "plain" tablefmt without header and line drawings.

[achimnol]

@miraliahmadli Places to explore:

• ai.backend.client.cli.admin.sessions
  → you should look here since ps is an alias to admin session command.
• ai.backend.manager.models.kernel to see which fields are available (SessionCommons class)

First, fire up a locally running Backend.AI cluster using the install-dev script.
And then run several sessions (just several times of backend.ai run --rm -c "print('hello world')" lablup/python:3.6-ubuntu18.04 would be sufficient) so that you have some list of history to tinker with the client.

[achimnol]

Also I've transferred this issue to lablup/backend.ai repository, since we are now gathering all Backend.AI-related issues in one place. (Existing links will be automatically redirected.)