[FIX] Packaging: RedHat: install in /usr and add systemd unit file

By default, the package was installed in /usr/local/lib/python2.7 in CentOS7.
It is problematic because most of other python packages are installed in /usr/lib
and also because /usr/local/lib is not in the default PYTHONPATH. It's fixed by
overriding the install script and forcing prefix to be /usr.

Added a minimal systemd unitfile.
Adapted tests accordingly.