Update repository structure

[simonedavico]

We need to update the repository structure to decouple implemented plugins from an empty template for the development of a new plugin.

The idea is that implemented plugins will have this structure:

plugins/
    camunda/
         v1-v3/
              WfMSPlugin.java
         v4/
              WfMSPlugin.java
         myVersion/
              WfMSPlugin.java

Short explanation:

• Each WfMS has its own folder;
• Each subfolder contains a plugin for a version or a range of versions;
• Each plugin must be named WfMSPlugin.java, since this is the structure drivers-maker expects (see drivers-maker/#24).

In addition to the plugins folder, we will also add the template for creating a new plugin (plugin-template), which will be a Maven project that a plugin implementor can download and use as starting point. This makes easy for us to augment Maven's lifecycle to automatically copy the implemented plugin in the appropriate directory before a pull request.

We assume that all the folders and files are lowercase (e.g., relevant for benchflow/data-analyses-scheduler#83)