Feature: Added a mail server for ADOP Containers SMTP

[bzon]

Works well with my other Pull Request "addition of groovy to configure SMTP in Jenkins" -Accenture/adop-jenkins#17

@anton-kasperovich @nickdgriffin For your review.

Based from https://hub.docker.com/r/tvial/docker-mailserver/

Pre-requisite before deploying the mail server..

  mkdir mail-config
  docker run --rm \
                     -e MAIL_USER=${INITIAL_ADMIN_USER}@${LDAP_DOMAIN} \
                     -e MAIL_PASS=${INITIAL_ADMIN_PASSWORD_PLAIN} \
                     -ti tvial/docker-mailserver:v2 \
                     /bin/sh -c 'echo "$MAIL_USER|$(doveadm pw -s SHA512-CRYPT -u $MAIL_USER -p $MAIL_PASS)"' >> mail-config/postfix-accounts.cf

  docker run --rm \
                     -v "$(pwd)/mail-config":/tmp/docker-mailserver -ti tvial/docker-mailserver:v2 generate-dkim-config

If approved, this new environment variables must be added in Jenkins container..

SMTP_USER: "${INITIAL_ADMIN_USER}@${LDAP_DOMAIN}"
SMTP_PASSWORD: "${INITIAL_ADMIN_PASSWORD_PLAIN}"
SMTP_HOST: "mail-server"
SMTP_PORT: "25"
SMTP_SYSAD_EMAIL: "jenkins@${PUBLIC_HOSTNAME}"
ADOP_SMTP_ENABLED: "true"