Create functional defaults for Genegraph environment variables

[tnavatar]

In order to run Genegraph, one requires specific environment variables to be set. These should have default values that allow Genegraph to run without prior configuration. If the current codebase makes this impossible, the code should be altered to make it possible.

Suggested alternatives for current variables:

*** GENEGRAPH_DATA_PATH

Default to directory within codebase

*** GENEGRAPH_DATA_VERSION

Default to generic public migration

*** GOOGLE_APPLICATION_CREDENTIALS

Should find a way to render unnecessary 

*** GENEGRAPH_BUCKET

Default to public bucket

*** CG_SEARCH_TOPICS

Default to empty string

*** GENEGRAPH_EXPERIMENTAL_SCHEMA

May be ready to obsolete this altogether

*** GENEGRAPH_BATCH_EVENT_SOURCES

Default to empty string

*** DX_JAAS_CONFIG

May be unavoidable, but could consider a way to address