Werk #15079: Change log level of RRD error messages (cmc.log)

In several places in the UI (e.g. the "IT infrastructure efficiency" views) a lot of unnecessary logging to cmc.log could occur. Missing rrd data would yield error messages such as Error accessing RRD: don't understand 'some_metric'. The log level for this specific case is changed from "Warning" to "Informational", so the mentionend error messages do not appear in cmc.log by default anymore.

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