Fixed usage of a different mirror that the last update

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Related tickets | #5
| License       | GNU GPLv3

Try to follow `setup.exe` behaviour for the `setup.ini` file.

The `setup_ini`  configuration field is not used any more.

- [x] Added back compatibility layer
- [x] Added a note that for the version 2.0 `cyg-apt` will not use anymore `/etc/setup/setup.ini`
- [x] Improved news notes
- [x] Added a deprecation warning when the `setup_ini` config field is used
- [x] Waiting for #53 because of an untested feature ([ref](https://github.com/alquerci/cyg-apt/blob/5285fccc451e7b8eb41754cc429fe8d681bce77d/src/cygapt/setup.py#L250))

closes #5