adds first work on mysql

This commit switches the basic setup by ncadou/docker-email-server from
sqlite to mysql connection. Connection parameters are set via
ansible/vars/defaults.yml. Also, spamassassin is set do "ENABLED=1" for
starting correctly.
changes ansible style to playbook

This commit changes the complete structure according to
http://docs.ansible.com/playbooks_best_practices.html#directory-layout

The mailserver to build is now in the group "mailservers" and belongs to
the roles "common" and "mailserver". Tasks, variables, handlers, files
and templates for the roles are now defined in "roles".

Legacy style ${var} have largely been replaced by {{ var }}.

There is a single configuration file for private data in
ansible/host_vars which has to be filled with mysql connection
parameters. The file "mail.localhost" would build a mailerver with that
hostname, exclusively - so this filename needs to be changed.
Accordingly, the new hostname has to be given in ansible/hosts.

Dockerfiles was modified to call site.yml
Fixes communication with MySQL

This commit introduces some changes that fix the communication between
dovecot, postfix and MySQL. Also, a new switch is added to create the
necessary database entries (CREATE_MYSQL_SCHEME). Use only once.

A new ansible variable "smtphost" is added to account for different
smtp domains (e.g. smtp.example.com).
Further fixes.

This commit changes the reject lists to use spamhaus' zen list. Also,
further configuration changes to postfix are made.

The role "common" is removed, we are in a single-instance setup here. No
further roles necessary.

Since the mail direcotry is an exposed volume, docker mounts it's
contents as root. Therefore, the startup-script will chown all
directorys under $VMAIL_DIR with 770 and all files with 660.
removes ViMbAdmin, cleans ansible setup

This commit removes ViMbAdmin, the nginx server and the exposed port 80.

Furthermore, the unnecessary tasks in packages.yml are removed, as we
install packages via Dockerfile.

Postfix now takes mail from multiple networks via host_vars. In there, a
YAML list "postfix_add_mynetworks" can be modified to inject multiple
mynetworks to postfix's main.cf.