RFC: Support multiple versions of the same service in the TT file

[piojo-zz]

So, if we have a service whose configuration format changes dramatically (say, logstash from 1.0 to 1.2), we need to be able to generate the different formats.

@stdweird proposes to have, inside each service that has this issue, a sub-directory per version. For those services that exist already, we'd have a top-level main.tt file that just includes the main.tt for that version.

For instance, in Logstash:

logstash/main.tt -> logastash/1.0/main.tt
logasth/1.0/main.tt
logasth/1.4/main.tt

this allows for ever-changing file formats and yet to keep backwards compatibility.

[stdweird]

The current multi-version behaviour goes as follows:
If there is one or more subdirectory in the service that start with an integer, it is assumed that this service is versioned.
The tests are then searched and executed per verion.

The pan directory is unique for the service not for the versions. Making backwards compatible templates is the only way forward.

TT-files can be shared between versions (they inlcuee using a relative path that holds the version anyway).