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.
Merge pull request #464 from camptocamp/fix-getfeatures_group2

Fix query layer search.