[trees] - install dovecot from debian backports to achieve maildir compatibility
requested to merge neutron/trees:install_dovecot_dependencies_from_backports_to_get_maildir_working into master
- adds compatibility for dovecots maildir
- adds role for installing debian backports
- installs newer version of dovecot 2.2.* from debian backports
- in newer versions of dovecot >= 2.2.31 cache index corruption is fixed
- adds note in README.md
Closes #24 (closed)