Add queryMode and logGroupNames to cloudwatch target

[alex94240]

Add queryMode and logGroupNames to cloudwatch target to allow search on cloudwatch logs (grafana 7).

With Grafana 7 we are now able to query the logs and display the result in a table pannel. We need some new fields:

• queryMode (Metrics/Logs)
• logGroupNames (needed when we are in Logs queryMode)

We also need fields like "namespace" and "metric" to become optional as we don't need them anymore when we are in Logs queryMode.

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