Werk #15723: Ensure site is stopped when running cmk-update-config

The cmk-update-config command, which is usually executed as part of omd update expects the site to be stopped during execution. This is always ensured during omd update.

However, the command can also be executed manually later on. In this situation the site can be running. To prevent unexpected results, the command now protects against an accidentally running site on invocation.

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