Werk #12876: Checking of output phases of UPSs and PDUs: Various fixes

This werk includes various fixes for the checking of output phases of UPSs and PDUs:

• The rule "Parameters for output phases of UPSs and PDUs" allows for configuring levels on the load of the output phases. However, the configured levels had no actual impact on the monitoring behavior.
• Any levels with a warning value of exactly 0 configured in the rule above had no impact on the monitoring behavior.
• Values excatly equal to upper thresholds configured in the rule above were still considered as below this threshold. This is inconsistent with the general behavior of Checkmk and has been fixed.

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