Capability detection for networker

In a first approach the decision on using the standard or the safe networker has been done based on the provider type. In case of all but MAAS the safe networker has been used. Now a new environment capability has been added, so that each provider can return if it requires a safe networker for the current machine. As this today depends on the machine (local provider dislikes the changes done by the networker on the bootstap node, because it's the host system) and if the machine has been manually provisioned.

Fixes #1354365.
How about MachineInfoGetter instead? The interfaces as defined does not imply it can be used only for RequireSafeNetworker capability. If you choose to rename it, I'd use the following doc comment: // MachineInfoGetter allows getting information about a machine, including its id and whether it was manually provisioned.