Add option to revert a snapshot before a job gets executed on a VM Node

This adds an extra dropdown to the node config page, with the same snapshot options that are displayed in the shutdown snapshot dropdown.

Selecting a snapshot in the new dropdown will activate the feature. Having no selection (the default value) will disable it. When active, every time a job starts on the node, the node will be cleanly disconnected, and the VM snapshot will be reverted before relaunching the node.

Because this relationship is between Jobs and Nodes, configuring more than one executor for a Node with this feature active is almost certainly going to mess things up, with the n+1 job reverting the snapshot while the n job is executing. Enforcing this in the UI is may be a good idea, but is not implemented yet.