Fix query layer search.

Now layersConfig will contains layers defined in the layer tree
(MapServer layer or group).
External Layers are stored in a deferent variable
(externalLayersConfig).
getQueryableWMSLayers return Mapserver Layer (no group) as object or
name.
getQueryableWMSLayers is not called for layer that defined
mapserverLayers or queryLayers.

Tested on Cartoriviera and on Morges
replace #447
fix #453 if we specify ```queryLayers: []``` on ortho layers