Improve girder transfers support

[axlbonnet]

At the moment, girder transfers are done through the girder-client binary which is a python package coming from pip.

But we have a python environment problem on many execution sites which causes this package installation to fail, and then also the transfer and the execution itself.

Several solutions are possible :

• install the package from cvmfs when possible, similarly to what's been done for boutiques
• replace the girder-client calls by curl requests (domnload and uploads are simple REST requests). We must be sure that curl is available on most sites (or wget ? or maybe use basic python modules ?)