[UX] Change the default `creation_policy` from `reuse_or_create` to `reuse`

[peterschmidt85]

Based on experience of using dstack apply, realized that creation of new fleets should always be explicit. By default, dstack should use the existing fleets. This makes it more predictable and much easier to explain.

To ensure the transition for existing users, in case dstack appy cannot find capacity and the no creation policy is specified, show a message: e.g.

Pass `--reuse_or_create` to ` dstack appy` or set `creation_policy` to `reuse_or_create` in the configuration."

[peterschmidt85]

The only problem with this so far is that we have backends that do not support fleets, incl. RunPod, Kubernetes, Vast.ai

[r4victor]

@peterschmidt85, I suggest we close this for now.