[UX] Allow to pass ssh instead of remote to backends in YAML config

[peterschmidt85]

Currently, if I want to run only on SSH fleets, I have to pass remote to backends in YAML config.
At the same time, if I use dstack ps or dstack fleet, it shows ssh in BACKEND.
Let's use ssh for consistency.

BTW, to double-check:
Ensure region for SSH fleets is None (instead of remote or empty string)

[jvstme]

Same for the --backend option in dstack apply and other CLI commands that have it