initialize_subcommand does not print help

IPython kernelspec is deprecated (ipapp.py) :

```
subcommands = dict(
...
kernelspec=('jupyter_client.kernelspecapp.KernelSpecApp',
"DEPRECATED: Manage Jupyter kernel specifications."
),
)

```

Initialize subcommand in traitlets though does not print it:

```
@catch_config_error
def initialize_subcommand(self, subc, argv=None):
"""Initialize a subcommand with argv."""
subapp, help = self.subcommands.get(subc) # help unused.

if isinstance(subapp, string_types):
subapp = import_item(subapp)

# clear existing instances
self.__class__.clear_instance()
# instantiate
self.subapp = subapp.instance(config=self.config)
# and initialize subapp
self.subapp.initialize(argv)
```

example

```
$ ipython kernelspec
No subcommand specified. Must specify one of: ['install-self', 'list', 'install']

Manage Jupyter kernel specifications.

Subcommands
-----------

Subcommands are launched as `jupyter kernelspec cmd [args]`. For information on
using subcommand 'cmd', do: `jupyter kernelspec cmd -h`.

install-self
[DEPRECATED] Install the IPython kernel spec directory for this Python.
list
List installed kernel specifications.
install
Install a kernel specification directory.
```

It default to the subapp description, which in itself is not deprecated, But its IPython alias is.

Does affect many subapp.