Werk #7769: Fix broken computation of host contact groups in some cases

The core config creation was broken in 1.6.0b1 when using the WATO folder hierarchy for assigning contact groups to hosts.

An exception like this was displayed after the update or later, e.g. when executing cmk -U to update the core configuration:

File "/omd/sites/produktiv/lib/python/cmk_base/cee/core_cmc.py", line 457, in cmc_all_hosts
cmc_hosts = CMCHosts(config_cache.all_active_hosts(), CMCHostConfig)
File "/omd/sites/produktiv/lib/python/cmk_base/cee/core_cmc.py", line 501, in __init__
self._compute(hostnames, host_class)
File "/omd/sites/produktiv/lib/python/cmk_base/cee/core_cmc.py", line 513, in _compute
host_config = host_class(hostname)
File "/omd/sites/produktiv/lib/python/cmk_base/cee/core_cmc.py", line 962, in __init__
super(CMCHostConfig, self).__init__(hostname)
File "/omd/sites/produktiv/lib/python/cmk_base/cee/core_cmc.py", line 753, in __init__
self._host_contact_groups = self._host_config.contactgroups
File "/omd/sites/produktiv/lib/python/cmk_base/config.py", line 2471,
in contactgroups
return list(set(cgrs))
TypeError: unhashable type: 'list'

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