Name and numbering convention for the VM images and the vagrant config for those who want the latest updates

Issue Background

Before starting to make VM images available, it in neccessary to define a naming / numbering convention.

Issue description

• Define a name / numbering convention for the images made available to the public.
• Only stable releases and pre-releases will be made available to users. For those interested in the latest changes, we provide VMs through Vagrant.

Acceptance criteria

• Link to the Release howto page where the naming / numbering convention is described as part of the release process.

This task blocks #37 (closed)