Networker mode based on Jobs

So far the Networker has been started with `NewNetworker()`, only for the MAAS provider the starting function is `NewSafeNetworker()`. The difference is that the regular variant does change networking configuration files on the machine, while the safe variant doesn't. To avoid the impression that the first variant is unsafe the starting has been changed to only `NewNetworker()` with an argument `intrusiveMode`.

This intrusive mode is the standard, except when

- the provider is MAAS, or
- the provider is the manual provider, or
- the provider is the local provider and the machine is the bootstrap, or
- the machine is manually provisioned, or
- the environment configuration `disable-network-management` is set to `true`.

This PR doesn't cover upgrading yet.