Werk #7970: Fix possible AttributeError "function" has no attribute "get" exception during checking

In the moment a check timeout occured while checking a host when using the Microcore an exception like this could occur in all previous 1.6 releases:

An exception occured while processing host "beta"
Traceback (most recent call last):
  File "/omd/sites/beta/lib/python/cmk_base/cee/keepalive.py", line 331, in do_keepalive
    exit_code = host_config.exit_code_spec.get("timeout", 2)
AttributeError: 'function' object has no attribute 'get'

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