cloud-init clean does not remove /etc/network/interfaces.d/50-cloud-init.cfg

[Sirio Balmelli]

`cloud-init clean` does not remove `/etc/network/interfaces.d/50-cloud-init.cfg`

How to reproduce:

1. ubuntu 16.04.5

2. install a static cloud-init config containing network v2
e.g.

```
network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - "192.168.0.1"
```

3. Install a *new* cloud-init config which disables network
(this is because cloud-init network v2, among other bugs *ignores* the `metric` directive - but that is not this bug):

```
network:
  config: disabled
```

4. Run `cloud-init clean && cloud-init init`

5. The file `/etc/network/interfaces.d/50-cloud-init.cfg` is still there

6. Manually remove `/etc/network/interfaces.d/50-cloud-init.cfg` and run clean+init again ... it is not recreated.

This means that /etc/network/interfaces.d/50-cloud-init.cfg created under cloud-init is *not* deleted when running `cloud-init clean` == bug == unexpected behavior.

[Ryan Harper (raharper)]

Thanks for filing a bug. It's definitely a file cloud-init generates and it could clean it up if requested.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3240