debian: get rid of whole debian/changelog

Future changes to this file should kept only one entry and update the
version number. Changelogs for "unofficial" (as not from Debian)
packages are usually quite boring.

The version number is also chosen such that the package can be
upgraded to an "official" package if one becomes available.
debian: simplify debian/rules

Use *.install to install files in the correct locations instead of
using directly `install` command.

Remove unneeded comments at the top of the file.

Let debhelper invoke setup.py. Also, let exabgp be installed as a
regular Python package. It works fine this way and a user cannot
install several versions of the same Debian package. Moreover, this
avoids to update the version of ExaBGP in debian/rules at each update.

Let debhelper handles cleaning as well.
debian: remove exabgp.substvars which is a generated file

This file is generated from dh-python2 helper.
debian: remove exabgp.dirs

This file is not needed as the directories listed here are created
when needed by other helpers.
debian: install examples with dh_installexamples
debian: install systemd files in a location expected by systemd in Debian