Feature/add config override

[anilnatha]

Purpose

From time to time Unity configuration settings need to be overridden, especially in Jupyter environments when testing Unity services.

Proposed Changes

• [ADD] Ability to override default Unity configuration settings by passing in a config file containing the settings to override when instantiating a Unity object.

  Issues

• https://github.com/unity-sds/unity-py/issues/56

  Testing

• Added unit test with test configuration file to assert that overriding settings works as intended.
• Tested in jupyter environment

[coveralls]

coverage: 86.041% (+0.03%) from 86.009% when pulling e1a1fd4e68e1739d5eefdda0f3e73a9592894402 on feature/add-config-override into d94ab53db1f0d4b618964c65a04839921b6b0814 on develop.