Break out deprecated app_engine arg into its own resource/module (v2.0)

[nstogner]

The app_engine field in the google_project resource is deprecated.

Options:

1. Remove app engine functionality from project factory
2. Integrate the app engine resource into project factory

This PR is for option 2.

Option 2 requires removing the app_engine arg and replacing with more granular args. This is needed b/c of terraform's limitation in accessing nested map fields (app_engine["feature_settings"]). Note: Option 2 also requires the user to terraform import the new resource as a part of the upgrade path, ex:

terraform import module.project-factory.module.project-factory.module.app-engine.google_app_engine_application.app pf-ci-test-full-y2d5b2ae-0d6c

[adrienthebo]

Noting for posterity that this is a backwards incompatible change since it changes the Project Factory interface.

[morgante]

Looks like this is causing tests to fail. Please investigate.

[Zest-nikunj]

@nstogner Whenever I try to run a terraform plan with the module, I get an error:
project-factory.google_project.main: "app_engine": this field cannot be set
Any suggestions?

[mikeraimondi]

I am also getting the error "project-factory.google_project.main: "app_engine": this field cannot be set" with v2.0.0 of the Google provider.

[morgante]

Replaced with #144.

[Zest-nikunj]

I am also getting the error "project-factory.google_project.main: "app_engine": this field cannot be set" with v2.0.0 of the Google provider.

I had to comment the app_engine variable, https://github.com/terraform-google-modules/terraform-google-project-factory/blob/master/modules/core_project_factory/main.tf#L93
to make it work. @mikeraimondi