Vagrant specification for a standalone MusicBrainz-Server dev instance

The musicbrainz-server/Vagrantfile is able to bring up a Ubuntu Precise
virtual machine up, which has been provisioned to run PostgreSQL, memcached,
Redis and MusicBrainz-Server.

Also, admin/import-db.sh will import the LATEST mbdump*.tar.bz2 archives into
the virtual machine. Thus from a fresh checkout, just running
admin/import-db.sh will import the virtual machine, provision it and import
data.
Use RT_SLAVE for the musicbrainz-server VM
Clarifying README with comments from review