[NRRPLT-2727] Add black material for gzweb

Change-Id: Ic4a8bb8d6cac74d5dd1a463da02a77c409da1eef

http/client/assets/materials/black.material

+material Gazebo/Black 
+{ 
+  technique 
+  { 
+    pass ambient 
+    { 
+      ambient 0 0 0 
+      diffuse 0 0 0 
+      specular 0.1 0.1 0.1 1 5
+    } 
+  } 
+}