Avatar: do not use default URL for gravatar

gravatar will serve default avatars itself, which do not work when the bot is in an intranet
We'd better use same default as github (retro)

master/buildbot/www/avatar.py

@@ -31,11 +31,16 @@ def getUserAvatar(self, email, size, defaultAvatarUrl):
 
 class AvatarGravatar(AvatarBase):
     name = "gravatar"
+    # gravatar does not want intranet URL, which is most of where the bots are
+    # just use same default as github (retro)
+    default = "retro"
 
     def getUserAvatar(self, email, size, defaultAvatarUrl):
         # construct the url
         gravatar_url = "//www.gravatar.com/avatar/"
         gravatar_url += hashlib.md5(email.lower()).hexdigest() + "?"
+        if self.default != "url":
+            defaultAvatarUrl = self.default
         gravatar_url += urllib.urlencode({'s': str(size), 'd': defaultAvatarUrl})
         raise resource.Redirect(gravatar_url)