[gae.py] Add a hacky support for parameterizing GAE YAMLs.
[gae.py] Add a hacky support for parameterizing GAE YAMLs.

We need something to avoid mistakes when deploying GAEv2 apps. They
consume configuration via flags which currently must be hardcoded
into YAMLs. There's only one YAML, but there are multiple
environments it can be deployed into. This CL adds a very adhoc way
of parameterizing YAML based on where it is being deployed to.

Should be superseded by a real GAE deployment automation.

Usage example:

luci_gae_vars:
  commit-queue-dev:
    AUTH_SERVICE_HOST: chrome-infra-auth-dev.appspot.com
  commit-queue:
    AUTH_SERVICE_HOST: chrome-infra-auth.appspot.com

entrypoint: >
  /usr/local/bin/start
  -auth-service-host ${AUTH_SERVICE_HOST}
  ...

Note that trying to use such YAML directly with
"gcloud app deploy" or with older version of gae.py fails.

R=tandrii@chromium.org

Change-Id: I06aa538c8fc4a73ebc79fd9ec0b066edfb0a86b7
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2065424
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py