Add option to pick whether to keep or decompose machines in a VM host

[Alberto Donato]

We recently landed a change in MAAS which drops the distinction for VM hosts between pre-existing VMs and those composed by MAAS.
With this change, MAAS won't decompose (i.e. destroy) VMs by default when removing a VM host through the API or the UI.

The change in https://code.launchpad.net/~ack/maas/+git/maas/+merge/396937 adds a boolean "decompose" flag which can be passed to the websocket call for deleting a pod to request for *all* VMs in a pod associated to MAAS machines to be deleted on removal.

The UI should add an option to allow this behavior when removing a VM host.