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.
This test is wrong - it should check for not found error not being masked, and there should be a separate test for unauthorized. See other examples how you can combine a "normal" (i.e. no error, we get results), "not found" but authorized (i.e. you've logged in as the agent of machine 1, but trying machine-1-lxc-0 should give you not found, rather than unauthorized), and "unauthorized" (i.e. "machine-42")