Add support for grouping hosts based on host labels

[robin-checkmk]

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What is the current behavior?

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Host labels can neither be read as a variable, nor be used as a group source.

What is the new behavior?

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• Add labels as a source for groupsources which creates one group per host label, named label_<key>_<value>. Characters that are not valid in a group name are replaced by underscores.
• The labels of a host are now available as the host variable checkmk_labels, whether or not labels is listed in `groupsources``. The effective labels are read from the monitoring core, so they include the labels Checkmk assigns through label rules and service discovery and not only the ones configured in Setup.

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