get of a nested config property should print a list of items

[gronke]

It would be nice to get a bunch of variables when querying a nested BaseConfig property with the ioc get CLI command. Printing multiple values is already supported (see --all), so this enhancement would allow to select an entire namespace in their output.

Nested sub-namespaces are expected to work in the same way.

Current result

$ ioc get provisioning. webirc
The config property 'provisioning.' is unknown
Traceback (most recent call last):
  File "/usr/local/bin/ioc", line 10, in <module>
    sys.dd:exit(cli())
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/src/ioc/ioc_cli/get.py", line 97, in cli
    value = lookup_method(source_resource, _prop)
  File "/usr/local/src/ioc/ioc_cli/get.py", line 131, in _lookup_jail_value
    value = resource.getstring(key)
  File "/usr/local/lib/python3.6/site-packages/libioc/Resource.py", line 356, in getstring
    value = self.get(key)
  File "/usr/local/lib/python3.6/site-packages/libioc/Jail.py", line 229, in get
    return self.jail.config[key]
  File "/usr/local/lib/python3.6/site-packages/libioc/Config/Jail/JailConfig.py", line 86, in __getitem__
    return super().__getitem__(key)
  File "/usr/local/lib/python3.6/site-packages/libioc/Config/Jail/BaseConfig.py", line 571, in __getitem__
    self._require_known_config_property(key)
  File "/usr/local/lib/python3.6/site-packages/libioc/Config/Jail/BaseConfig.py", line 806, in _require_known_config_property
    logger=self.logger
libioc.errors.UnknownConfigProperty
$ ioc get provisioning.source webirc
/usr/local/etc/puppet