Improve data prefix (file:)

[axlbonnet]

Currently, everything work properly if dealing only with lfn: path.

But when dealing with file: or girder: paths, there are issues on several points :

• on relaunch, the path is broken with a "file:" in the middle
• with girder (and file?), the paths in the inputs/outputs of an execution are not good

There is already a property in vip.conf to select if we use lfn (default) or file.
Depending on the mode, it should be the default everywhere and it should never be necessary to add it on launch or relaunch.
It should also be used to select the storage location at import.