2 bugfixes (IPMI): take return value of ipmi_sensor_read into account; close sdrCache + ipmi-connection after reading

Adjust PDUUnit::refresh() to actually work with Megware Clustsafes

See merge request ga96sur/dcdbpusher!1

 - dont cache HTTPS response in file
 - catch exception if value could not be read

Instead of using 2 specific IDs, a generic path to the value in the XML-file has to be given now.
This enables the plugin to also be used for other XML use cases apart from specific PDU-case.

Further testing required. Remove test code afterwards

Missing:
-Certificate validation and so on required?
-just send request.xml? or use GET or something?
-undefined symbol error when trying to load plugin