Ansible role for a basic dovecot instance added.

Basic dovecot role with these functionalities as described on the README.md file:

• Virtual users from LDAP.
• Non encrypted communication between Dovecot and LDAP.
• mbox type mail boxes stored in /var/mail and accessed by mail user.
• No LDAP admin user needed for look-ups.

This role does not include an MTA (like Postfix or Exim) that will be necessary to have communication between 2 users.