In object template, use variable in fbxModel attribute

We want to have "fbxModel"attribute as an URI in rack/device/generic template, with a FBXROOT variable defined in config.toml.

fbxModel is now a proper URI (OGrEE-3D can load the .fbx file from given file or from given url)
if FBXROOT is not defined / empty / an non-existing path, default is used
default FBXROOT is https://github.com/ditrit/OGrEE-3dModels

ditrit / OGrEE-3D