[WebAPI] Adds the uCernVM Version in the configuration file

* Adds the vmcp directory in the source tree. ``sign.php`` was modified to read the ``ucernvm_version`` from the GET arguments
* ``config.example.py`` was updated with the ``WEBAPI_UCERNVM_VERSION`` variable that holds now the  default uCernVM version to be used when a user launches an instances with WebAPI. It can be overridden by the various configurations by adding ``ucernvm_version`` key in the respective dictionary (see ``src/cvmo/config.example.py`` if confused).