Integration tests for FINOS Legend Engine

[claudiubelu]

Currently, we're only running lint and unit tests on presubmit. Unit tests are useful, but insufficient in ensuring that new pull requests do not break the current charm.

The integration tests should:

• build and deploy the charm.
• add the necessary (tester) charms and relations for the following relations: legend-db (receives mongodb credentials), legend-studio-gitlab (receives gitlab's secret name and secret ID needed by Legend).
• check that the charm becomes active and that Legend Engine is reachable through its IP (http://pod_ip:6060).
• check that the Legend Engine configuration files are updated when the above relations are updated with new data.
• check that the charm updates the legend-engine-gitlab relation when its external-hostname and / or enable-tls config option is changed.
• check that the charm updates the legend-engine relation when its external-hostname and / or enable-tls config option is changed.