musicbrainz-server: New cookbook to provision musicbrainz-server instances

This commit introduces the 'musicbrainz-server' cookbook, which provides recipes
for provisioning musicbrainz-server instances. There are two recipes:

* musicbrainz-server::install - this recipe "installs" musicbrainz-server, by
  performing a Git clone and installing all Perl dependencies from my PPA.

* musicbrainz-server::production - this recipe extends the ::install recipe by
  configuring nginx as a reverse proxy to a daemontools-backed FastCGI socket.

There is a templated DBDefs.pm which uses attributes to determine how to expand
this template.