Werk #10038: Fix cmk/check_mk_server:yes label related issues

This label was detected based on the asynchronously created agent section <<<omd_status>>>. Because this section is not immediately available on the first agent contact, this resulted in a label discovery after the second check cycle.

Another issue was that the <<<omd_status>>> section was produced by non Checkmk servers with older agent versions. This lead to a wrong label discovery.

We now use the dedicated <<<labels>>> section to create the label which is produced once we find the command 'omd' on a system.

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