Add support for Fedora / RedHat / Centos

Created by: nemesisdesign

Fedora is a popular linux distribution that is suitable to be used for hosting an openwisp2 instance.

The parts to change to add support to Fedora are the following ones:

• management of system packages: use yum module instead of apt and the names of some packages are different
• nginx configuration, path differ slightly
• supervisord configuration, a few things differ here too