Werk #1731: ipmi_sensors: Fixed agent part when ipmi-sensors call on first agent run

The agent code always failed when ipmi-sensors is executed with --sdr-cache-directory /var/cache for the first time using the agent. This happens because the code tries to create a locked file within the directory /var/cache/.freeipmi/sdr-cache while this directory does not exists. The agent now ensures the existance of this directory.

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