Introduce Fleets for cluster provisioning and management

[r4victor]

Currently, dstack provides the dstack pool add command to provision cloud instances and the dstack pool add-ssh command to add on-prem instances via ssh. This interface has several limitations. The major one is that there is no way to provision/add clusters. Managing clusters via pools is also not optimal.

The proposal is to introduce fleets for provisioning and managing instances and clusters of instances. Fleets are to be provisioned with dstack apply. Here's what fleet configuration will look like:

# cloud fleet
type: fleet
name: my-fleet
nodes: 4
placement: cluster/any
resources:
# ... all profile params

# ssh fleet
type: fleet
name: my-ssh-fleet
ssh:
  user: ubuntu
  ssh_key: ~/.ssh/key.pem
  port: 22
  network: "1.0.0.0/24"
  hosts:
    - "1.1.1.1"
    - "2.2.2.2"
    - hostname: "3.3.3.3"
      user: different-user
    - hostname: "4.4.4.4"
      ssh_key: ~/.ssh/different-key.pem

[peterschmidt85]

type: instance or cluster? :)

[r4victor]

I'd start with instance since it's what dstack pool currently works with. We can discuss how dstack supposed to work with clusters in a separate issue: we could expand instance configuration to be able to provision multiple cluster nodes (may be as simple as specifying nodes: 8) or introducing a separate cluster configuration.

[peterschmidt85]

I wouldn't rush and at least discuss this before implementing.

[r4victor]

The description is updated to use fleet configuration type.