Add support for the upcoming Debian Stretch

Created by: xmaysonnave

While installing ansible-openwisp2 on Debian Stretch RC1 I face an uwsgi crash as ansible fetched uwsgi 2.0.14 from the PyPi repo. The crash is related to an OpenSSL 1.1 incompatibility.

However while digging and experimenting, Debian successfully installed uwsgi from its own repo. In order to have a functionnal installation I modified the file :

/etc/ansible/roles/openwisp.openwisp2/tasks/pip.yml

with the following informations :

- name: Install uwsgi
  pip:
    name: http://http.debian.net/debian/pool/main/u/uwsgi/uwsgi_2.0.14+20170111.orig.tar.gz
    state: latest
    virtualenv: "{{ virtualenv_path }}"
    virtualenv_python: "{{ openwisp2_python }}"
    virtualenv_site_packages: yes
  with_items:
    - uwsgi
  notify: reload supervisor

That way pip fetch, build and doesn't complain.

Thanks