Add VM ordering for startup / shutdown

Via configuration syntax like:

```
[vm.ros-vm1]
before = ["ros-vm2"]

[vm.ros-vm2]
after = ["ros-vm1"]
requires = ["ros-vm1"]
```

Add explicit ordering and dependencies between VMs.

- after and before will be ordering based directives, where state of the configured VM isn't checked.
- requires will check if the set VM is up, and if configured ssh available.
