[2.x] Storing settings over the UI uses different form than API, which could cause API fields to be reverted

There was an error in MAAS where a global config was set to a default, but the actual value of the config was empty in the database. So when MAAS tried to use the setting, it would fail because it was null. This was found as part of [1], and because of [2].

The issue was that the UI would reset a global setting value that's not exposed in the UI, e.g:

1. maas <user> maas set-config name=maas_proxy_port value=8000
2. Go to UI and disable proxy
3. Go to UI and set 'built-in proxy'
4. maas <user> maas get-config name=maas_proxy_port would return 'null'

After investigating the settings, the API uses a different form from the UI. For example, the API validates the data from:

src/maasserver/forms/settings.py

While the UI uses a different form:

src/maasserver/forms/__init__.py:ProxyForm

Once I removed the field from src/maasserver/forms/__init__.py:ProxyForm, the UI stopped overriding the values of the field.

Based on the behavior observed this affects to the ProxyForm because the UI is in Angular, and no longer uses the settings form for validation. In comparison, the DNSForm does becuase the settings for this form are created automatically.

[1]: https://bugs.launchpad.net/maas/+bug/1787381
[2]: https://bugs.launchpad.net/maas/+bug/1787381/comments/6