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.
Those sort of doc comments are not really helpful, better not have anything than saying "2+2=4" basically :)
How about:
// SetMachineAddresses sets the addresses for each given machine tag and list of addresses.
And:
// GetMachines returns information about each given machine tag.