Using `--bootloader=none` causes an odd crash

When I run Subiquity with the `--bootloader=none` option, it crashes.

Steps:

- load an Ubuntu 18.04.3 live image
- switch terminal
- run `snap stop subiquity`
- run `snap run subiquity --bootloader=none`
- tap Enter twice
- crash!

If I run without the `bootloader` option, it runs fine.

I'm on an x86-64 machine.

Stack trace:

```
  File "/snap/subiquity/1093/usr/bin/subiquity", line 11, in <module>
    load_entry_point('subiquity==0.0.5', 'console_scripts', 'subiquity-tui')()
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquity/cmd/tui.py", line 161, in main
    subiquity_interface.run()
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquitycore/core.py", line 536, in run
    self.common['loop'].run()
  File "/snap/subiquity/1093/usr/lib/python3/dist-packages/urwid/main_loop.py", line 286, in run
    self._run()
  File "/snap/subiquity/1093/usr/lib/python3/dist-packages/urwid/main_loop.py", line 384, in _run
    self.event_loop.run()
  File "/snap/subiquity/1093/usr/lib/python3/dist-packages/urwid/main_loop.py", line 788, in run
    self._loop()
  File "/snap/subiquity/1093/usr/lib/python3/dist-packages/urwid/main_loop.py", line 825, in _loop
    self._watch_files[fd]()
  File "/snap/subiquity/1093/usr/lib/python3/dist-packages/urwid/main_loop.py", line 232, in cb
    rval = callback(data)
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquitycore/core.py", line 311, in in_main_thread
    callback(fut)
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquity/controllers/keyboard.py", line 63, in _done
    self.signal.emit_signal('next-screen')
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquitycore/signals.py", line 40, in emit_signal
    urwid.emit_signal(self, name, *args, **kwargs)
  File "/snap/subiquity/1093/usr/lib/python3/dist-packages/urwid/signals.py", line 265, in emit
    result |= self._call_callback(callback, user_arg, user_args, args)
  File "/snap/subiquity/1093/usr/lib/python3/dist-packages/urwid/signals.py", line 295, in _call_callback
    return bool(callback(*args_to_pass))
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquitycore/core.py", line 364, in next_screen
    self.select_screen(self.controller_index + 1)
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquitycore/core.py", line 353, in select_screen
    controller.default()
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquity/controllers/zdev.py", line 647, in default
    self.ui.set_body(ZdevView(self))
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquity/ui/views/zdev.py", line 147, in __init__
    self.refresh_model_inputs()
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquity/ui/views/zdev.py", line 157, in refresh_model_inputs
    self.zdev_list.refresh_model_inputs()
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquity/ui/views/zdev.py", line 74, in refresh_model_inputs
    zdevinfos = self.parent.controller.get_zdevinfos()
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquity/controllers/zdev.py", line 669, in get_zdevinfos
    return self.lszdev()
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquity/controllers/zdev.py", line 672, in lszdev
    devices = run_command(lszdev_cmd, universal_newlines=True).stdout
  File "/snap/subiquity/1093/lib/python3.6/site-packages/subiquitycore/utils.py", line 120, in run_command
    env=_clean_env(env), **kw)
  File "/snap/subiquity/1093/usr/lib/python3.6/subprocess.py", line 423, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/snap/subiquity/1093/usr/lib/python3.6/subprocess.py", line 729, in __init__
    restore_signals, start_new_session)
  File "/snap/subiquity/1093/usr/lib/python3.6/subprocess.py", line 1364, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'lszdev': 'lszdev'
```