Use fully qualified URL to GCE metadata server

The http://metadata shortcut only works on Linux, Windows requires
https://metadata.google.internal.

Bug: 738255
Change-Id: I06b6c34761bbee9912a8cd2a65ab43b3ae230f07
Reviewed-on: https://chromium-review.googlesource.com/611245
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: smut <smut@google.com>

gerrit_util.py

@@ -228,8 +228,8 @@ class GceAuthenticator(Authenticator):
   """
 
   _INFO_URL = 'http://metadata.google.internal'
-  _ACQUIRE_URL = ('http://metadata/computeMetadata/v1/instance/'
-                  'service-accounts/default/token')
+  _ACQUIRE_URL = ('%s/computeMetadata/v1/instance/'
+                  'service-accounts/default/token' % _INFO_URL)
   _ACQUIRE_HEADERS = {"Metadata-Flavor": "Google"}
 
   _cache_is_gce = None