'allowed_nodes' in ComputeScope does not work as intended

[Pavel Specian]

When availability_zone is defined in compute scope, this part of the code is called:
https://opendev.org/openstack/watcher/src/commit/bf69546224f248ba81c1d4114c62697be3751e97/watcher/decision_engine/scope/compute.py#L156:

self._collect_zones(rule['availability_zones'],
                                    allowed_nodes)

This leads to:
https://opendev.org/openstack/watcher/src/commit/bf69546224f248ba81c1d4114c62697be3751e97/watcher/decision_engine/scope/compute.py#L82:

allowed_nodes.extend(service.host)

'service.host' returns a hostname(str) which when used with extend() on an allowed_nodes list leads to something like this:

>>> x = []
>>> host = "host.example.com"
>>> x.extend(host)
>>> print(x)
['h', 'o', 's', 't', '.', 'e', 'x', 'a', 'm', 'p', 'l', 'e', '.', 'c', 'o', 'm']
>>>

This further breaks the logic of this:
https://opendev.org/openstack/watcher/src/commit/bf69546224f248ba81c1d4114c62697be3751e97/watcher/decision_engine/scope/compute.py#L203