Werk #8841: Change internal mechanic of discovered host labels

The discovered host labels which were introduced during the first beta versions of Checkmk 1.6 had to be reworked for some reasons. The new implementation is now based on the Service discovery mechanism, which makes several things a lot easier for us.

Host labels are now discovered during the procedure of Checkmk service discovery. All discovery functions of the checks are able to either discover services and host labels.

• The "Check_MK Discovery" service tells you about new found host labels, just like about services.
• You can use the "Service discovery" page of WATO to view the discovered host labels. New discovered host labels now trigger a warning state by default. This state can be configured using the "Periodic service discovery" ruleset.
• The automatic activation function, that can be configured in the "Periodic service discovery" ruleset may be used to automatically add new host labels on a regular base.
• Whenever you execute a service discovery (via command line, GUI or API), the host labels will also be discovered.
• CEE: The DCD can immediately add host labels when you have enabled the service discovery for a DCD connection.

Check programmers: You may add something like this to your discovery functions for creating host labels:

    yield HostLabel(u"os", u"Windows")
    yield HostLabel(u"type", u"security")

This adds two host labels os:Windows and type:security to the host.

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