[chromeperf] always use app_engine.Credentials for sheriff config client
[chromeperf] always use app_engine.Credentials for sheriff config client

Context: https://yaqs.corp.google.com/eng/q/3352382365786701824

We will use app_engine.Credentials to keep the consistent behavior.

Bug: chromium:1120196
Change-Id: Ia394f3e64d0eb86a2bbc48754ec011bfbf02211f
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/4021671
Commit-Queue: Wenbin Zhang <45ad8a1ff616a38a2202fd35571df81897c5f14f@google.com>
Reviewed-by: John Chen <334fbfbb4df7c78f091ea1b77c69ca6ac731a3f3@chromium.org>
Reviewed-by: Leina Sun <4dc7485e04f6a3a1d05495d5781efc53aef710e0@google.com>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult