Description of problem:
If a CDS or HAProxy node breaks, it's impossible to delete it from RHUI. That's because rhui-manager tries to connect to that node to stop the relevant services, and if the connection is impossible, the deletion fails.

There should be a way to handle this situation so that rhui-manager just forgets about the node internally.

Version-Release number of selected component (if applicable):
RHUI 3

How reproducible:
Always.

Steps to Reproduce:
1. Add a CDS node.
2. Make the node unavailable. Break it, turn it off, whatever.
3. Try to delete the node in rhui-manager.

Actual results:
An unexpected error has occurred during the last operation.
More information can be found in /root/.rhui/rhui.log.

The log file reads:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/rhui/tools/shell.py", line 88, in safe_listen
    self.listen(clear=first_run)
  File "/usr/lib/python2.7/site-packages/rhui/tools/shell.py", line 127, in listen
    Shell.listen(self)
  File "/usr/lib/python2.7/site-packages/rhui/common/shell.py", line 186, in listen
    item.func(*args, **item.kwargs)
  File "/usr/lib/python2.7/site-packages/rhui/tools/screens/cds.py", line 47, in unregister
    InstanceScreen.unregister(self, cleanup_script, force=force)
  File "/usr/lib/python2.7/site-packages/rhui/tools/screens/instances.py", line 298, in unregister
    self._unregister(unregistered_instances, bash_script=bash_script, force=True)
  File "/usr/lib/python2.7/site-packages/rhui/tools/screens/instances.py", line 313, in _unregister
    self.run_sudo(instance, 'rm -rf /var/lib/puppet/ssl')
  File "/usr/lib/python2.7/site-packages/rhui/tools/screens/instances.py", line 547, in run_sudo
    return sudo(command, **sudo_kwargs)
  File "/usr/lib/python2.7/site-packages/fabric/network.py", line 639, in host_prompting_wrapper
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/fabric/operations.py", line 1095, in sudo
    stderr=stderr, timeout=timeout, shell_escape=shell_escape,
  File "/usr/lib/python2.7/site-packages/fabric/operations.py", line 909, in _run_command
    channel=default_channel(), command=wrapped_command, pty=pty,
  File "/usr/lib/python2.7/site-packages/fabric/state.py", line 393, in default_channel
    chan = _open_session()
  File "/usr/lib/python2.7/site-packages/fabric/state.py", line 380, in _open_session
    return connections[env.host_string].get_transport().open_session()
  File "/usr/lib/python2.7/site-packages/fabric/network.py", line 151, in __getitem__
    self.connect(key)
  File "/usr/lib/python2.7/site-packages/fabric/network.py", line 143, in connect
    self[key] = connect(user, host, port, cache=self)
  File "/usr/lib/python2.7/site-packages/fabric/network.py", line 565, in connect
    raise NetworkError(msg, e)
NetworkError: Timed out trying to connect to cds01.example.com (tried 1 time)

The node is still tracked in RHUI.

Expected results:
It should be possible to delete the node from RHUI without touching it.

Additional info:
Apparent workaround: remove the information about the node from /etc/rhui/cds.json / /etc/rhui/haproxy.json.