First additions of JobManageNetworking (or comments where it hasn't to be added).
Next step in controlled networker start, still problems with races conditions during testing.
Merge branch 'master' of https://github.com/juju/juju into networker-mode-based-on-agent-api
Machine agent networker test isn't flaky anymore.
Merge branch 'master' of https://github.com/juju/juju into networker-mode-based-on-agent-api
Merge branch 'master' of https://github.com/juju/juju into networker-mode-based-on-agent-api
Added some missing job handling.
Some more network management jobs added.
Merge branch 'master' of https://github.com/juju/juju into networker-mode-based-on-agent-api
Merge branch 'master' of https://github.com/juju/juju into networker-mode-based-on-agent-api
Changes after review by Dimiter.
Merge branch 'master' of https://github.com/juju/juju into networker-mode-based-on-agent-api
Final post-review changes.
Merge pull request #876 from TheMue/networker-mode-based-on-agent-api

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.