Allow configuration of max snapshots during install

[anmazzotti]

Depends on rancher/elemental-toolkit#2193

apiVersion: elemental.cattle.io/v1beta1
kind: MachineRegistration
metadata:
  name: fire-nodes
  namespace: fleet-default
spec:
    elemental:
      install:
        snapshotter:
          type: btrfs
          maxSnaps: 2

Note that the default (and minimum value) now is 2. This differs from the previous 4 for btrfs snapshotter (was driven toolkit side).

Also note that this covers install phase only.
I considered implementing it for reset as well, however since you will actually be able to define a different snapshotter config on reset, this will make machine drift in behavior, depending if they are freshly provisioned or reset, which is not desirable imho.

Changing snapshotter type on reset also has consequences, for example switching from btrfs to loopdevice will have serious effects on partition sizes. Changing the amount of maxSnaps will also affect space available, and repartitioning is something we do not support on reset.

[davidcassany]

LGTM