Resolve deprecation warning from pecan

Resolve the following warning raised from pecan.

```
DeprecationWarning: The function signature for
watcher.api.controllers.root.RootController._route is changing in
the next version of pecan.
Please update to: `def _route(self, args, request)`.
```

Change-Id: I7081cf956a8baa05cd70ced0496ca8192fff979e
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

watcher/api/controllers/root.py

@@ -109,7 +109,7 @@ class RootController(rest.RestController):
         return Root.convert()
 
     @pecan.expose()
-    def _route(self, args):
+    def _route(self, args, request=None):
         """Overrides the default routing behavior.
 
         It redirects the request to the default version of the watcher API
@@ -118,4 +118,4 @@ class RootController(rest.RestController):
 
         if args[0] and args[0] not in self._versions:
             args = [self._default_version] + args
-        return super(RootController, self)._route(args)
+        return super(RootController, self)._route(args, request)