[webapi] Fix URL building

The current implementation lost the path of the
configured platform URL.

beat/cmdline/webapi.py

@@ -79,9 +79,14 @@ class WebAPI(object):
         }
 
     def __build_url(self, path):
-        url = "{schema}://{host}{path}".format(
-            schema=self.parsed[0], host=self.parsed[1], path=path
-        )
+        platform = self.parsed.geturl()
+        if platform.endswith("/"):
+            platform = platform[:-1]
+
+        if path.startswith("/"):
+            path = path[1:]
+
+        url = "{platform}/{path}".format(platform=platform, path=path)
         return url
 
     def get(self, path):