LMTP class re-factoring

This refactors the module to fully support LMTP, by including `dovecot::lmtp` (similar to `dovecot::imap`). It includes parameters to setup the protocol's plugins and postmaster address, as well as a unix socket file path (currently modeled for use with Postfix). It also installed the appropriate package, and fails if 'lmtp' is not included in `dovecot::base::protocols` as it cannot work otherwise. There is a dependency on `dovecot::master` because Postfix user/group/mode parameters can be resolved.

This includes the styles PR, but can be rebased if necessary.