Werk #7226: LXC: Add basic support for Linux containers

The agent is now detecting when it is executed in a Linux container context and changes it's behaviour depending the environment.

In containers the agent currently does the following:

• lxc_container_cpu: This check is used instead of the normal CPU utilization check
• zfs filesystems are not excluded for the df section anymore
• kernel section is not processed, because it's the host systems kernel the agent reports information for
• drbd section is not processed for the same reason
• lnx_thermal section is not processed for the same reason

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