[AIRFLOW-XXX] Make Breeze The default integration test environment

[potiuk]

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[potiuk]

This change clearly separates two development environments we have - Local Virtualenv and Breeze. This was a prerequisite I agreed with Elena to work on improving the documentation as a whole - now after Breeze has gone through teething problems we can make it clear that it can and should be used for running integration tests and local virtualenv should be used for local development/running unit tests.

[milton0825]

LGTM apart from one small nit.