Werk #13144: Add builtin host label "cmk/site"

Component	Core & setup
Title	Add builtin host label "cmk/site"
Date	Sep 16, 2021
Level	Trivial Change
Class	New Feature
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.1.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

Every host now discovers a builtin host label cmk/site:[sitename].

This can be used in rule conditions to filter for hosts on specific sites

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