Add env variable to skip GceAuthenticator for Gerrit CLs

BUG=skia:5979

Change-Id: I063a79a3877f4ad778bfbcbca3ddee49e6504ede
Reviewed-on: https://chromium-review.googlesource.com/411386
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Ravi Mistry <rmistry@google.com>

gerrit_util.py

@@ -218,6 +218,8 @@ class GceAuthenticator(Authenticator):
 
   @classmethod
   def is_gce(cls):
+    if os.getenv('SKIP_GCE_AUTH_FOR_GIT'):
+      return False
     if cls._cache_is_gce is None:
       cls._cache_is_gce = cls._test_is_gce()
     return cls._cache_is_gce