Checkmk Werk 13234: Fix validation of label keys in ruleset conditions

ID: 13234 Title: Fix validation of label keys in ruleset conditions Component: Multisite Level: 1 Class: Bug fix Version: 2.1.0i1 If several keys were used for label conditions in rulesets, e.g. "foo:bar" and "foo:baz", the first label was silently overwritten by the last label. You will now get an error message if keys are used twice.