Create a systemd unit and scripts in order to automatize a lxc mailserver container launch

Summary:
The mail server is isolated into a lxc container. We want this mail server to be operational at startup

This requires two tasks:

• start the container mailserver through lxc-start
• add to iptables rules for NAT forwarding

We also provide a stop script to clean iptables.

Resolves T466.

Test Plan:
To evaluate the container status, we used 3 ways:

• try a telnet connection from an external server to see if the SMTP server (see below)
• lxc-top
• iptables -L | grep 10.0.3.8 (the IP of the mailserver container)

It has also been tested in Dwellers through systemd and works.

$ telnet -4 dwellers.nasqueron.org 25

Trying 212.129.32.223...
Connected to dwellers.nasqueron.org.
Escape character is '^]'.
220 mail.nasqueron.org ESMTP Postfix (Debian/GNU)

Reviewers: dereckson

Subscribers: Kaliiixx, dereckson

Projects: Operations sprint 0, Servers, Mail

Maniphest Tasks: T466

Differential Revision: http://devcentral.nasqueron.org/D102