Ability to add an abstract for each layer and for wms services

[Marc-AndreDaviault]

In the conf file, the tag 'description' is not working for the service-metadata and for layers even if it's allowed by the WMS spec.

wms = {
	...	
    service-metadata = {
        name = "WMS"
		description = " " 	# not working 
	... 		
layers = {
    layer_name = {
		description = " " 	# not working