Add python-setuptools as required preinstalled package

Add python-setuptools as required preinstalled package

On some kind-of-minimalistic  Ubuntu 18.04.1 extra python package `python-setuptools` was needed in order to compile from source the final DEB.

```
dpkg-buildpackage: info: source version 1.2.1
dpkg-buildpackage: info: source changed by Julien Fortin <julien@cumulusnetworks.com>
 dpkg-source --before-build ifupdown2
dpkg-buildpackage: info: host architecture amd64
dpkg-checkbuilddeps: error: Unmet build dependencies: python-setuptools
dpkg-buildpackage: warning: build dependencies/conflicts unsatisfied; aborting
```