Executor host arbitrary string fails

[David Garcia]

In mistral/mistral/config.py:266:

    cfg.HostAddressOpt(
        'host',
        default='0.0.0.0',
        help=_('Name of the executor node. This can be an any string '
               'name/identifier. It is not necessarily a hostname, '
               'FQDN, or IP address. It is also related to the "target" '
               'attribute of tasks defined in a workflow text. If "target" '
               'is defined for a task then the action of the task will be '
               'sent to one of the executors that have the same value in the '
               '"host" property.')

Entering a value that isn't a hostname / FQDN / IP results in mistral failing to start.

 Traceback (most recent call last):
  File "/var/lib/mistral/mistral/.tox/venv/lib/python3.7/site-packages/oslo_config/types.py", line 846, in __call__
    value = self.hostname(value)
  File "/var/lib/mistral/mistral/.tox/venv/lib/python3.7/site-packages/oslo_config/types.py", line 799, in __call__
    raise ValueError("%s is an invalid hostname" % value)
ValueError: executor_pool is an invalid hostname

Changing the type of config option to StrOpt solves the issue, mistral starts and tasks with the "target" attribute set to the string are sent to the correct executors.