Improve datasource tests

[rrey]

SUMMARY

We have bug reports on grafana_datasource which reveals that even if Grafana API don't return any error, we can still have a datasource not working.
Example: #204

To improve this situation we could:

• Try to implement what Grafana has in the WUI in the datasource edit page as the "Save & test" button. Have the module run this test to make sure that the datasource is actually working.
• Improve integration tests to deploy third parties like Prometheus to allow the previous point to be tested ni the CI.

ISSUE TYPE

• Feature Idea

COMPONENT NAME

grafana_datasource.py