* Add dagster-databricks package

This package is closely modeled off the dagster_aws.emr subpackage and
provides the databricks_pyspark_step_launcher resource and the
DatabricksRunJobSolidDefinition solid for running Databricks jobs.

* Reference Databricks docs in dagster-databricks configs module

* Move build_pyspark_zip into dagster_pyspark utils module

* Fix style/minor issues in dagster-databricks

Specifically:

- triple single quotes instead of triple double quotes for docstrings
- single quotes instead of double quotes everywhere else
- oneline docstrings where possible; start on same line everywhere else
- rename 'is_terminal' to 'has_terminated'
- use 'databricks_run_id' instead of 'run_id' for clarity
- make DatabricksJobRunner.client a property
- remove unnecessary blank lines

* Add references to Databricks storage docs in 'main' script

* Add comment explaining global vars in databricks_step_main.py

* Fix Python 2 issues in dagster-databricks

* Check invariants when setting up storage in Databricks job

* Fix dependencies in dagster-databricks/tox.ini

* Move 'secret_scope' field into inner credentials object to simplify Databricks storage

* isort dagster-databricks

* Add pylint to tox.ini for dagster_databricks

* Install dagster-databricks in 'make install_dev_python_modules'

* Reference GitHub issue for better storage setup in databricks_step_main.py

* Uncomment dagster-azure related config

* Replace assert_called_once with call_count for Python3.5 compat

* Fix lint errors in databricks.py

* Improve handling of libraries by including required libs by default

* Fix version to match other dagster libraries

* Specify supported_pythons to exclude Python 3.8 from dagster-databricks tests on buildkite

See https://github.com/dagster-io/dagster/issues/1960.

* Add README for dagster-databricks

* Install dagster-databricks in dagster-examples tox.ini

* Update snapshot test for dagster example using databricks

* Add API docs for dagster_databricks

* Add coveragerc for dagster-databricks