Move from GCE to GAE Flex

[Splaktar]

This would help us get into a containerized deployment while handling auto scaling and service restarts automatically. We should use the default nodejs runtime which is based on Node 6.9.x in order to get the best integration with Cloud Trace and Cloud Debug.

More on GAE Flex which just went GA: https://www.youtube.com/watch?v=sATG0OfdP4g&list=PLIivdWyY5sqKjIKJ7-bRSWanvclrR573n&index=26

[Splaktar]

Finally got the Hub running on GAE Flex in this branch.

It took so many small tweaks and adjustments, you'll have to read the commits to find them. Over 18 hours working on this today. /yawn