[2.0] get_dhclient_info can raise a ProcessLookupError which makes registration fail

[Blake Rouse]

2016-03-03 14:20:39+0000 [ClusterClient,client] Event-loop 'maas-1:pid=927' authenticated.
2016-03-03 14:20:39+0000 [ClusterClient,client] Event-loop 'maas-1:pid=927' handshake failed; dropping connection.
 Traceback (most recent call last):
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 393, in callback
     self._startRunCallbacks(result)
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
     self._runCallbacks()
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 588, in _runCallbacks
     current.result = callback(current.result, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1184, in gotResult
     _inlineCallbacks(r, g, deferred)
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1128, in _inlineCallbacks
     result = g.send(result)
   File "/usr/lib/python3/dist-packages/provisioningserver/rpc/clusterservice.py", line 539, in performHandshake
     registered = yield self.registerRackWithRegion()
   File "/usr/lib/python3/dist-packages/provisioningserver/rpc/clusterservice.py", line 507, in registerRackWithRegion
     interfaces, _ = get_interfaces_definition()
   File "/usr/lib/python3/dist-packages/provisioningserver/networks.py", line 28, in get_interfaces_definition
     new_definition = get_all_interfaces_definition()
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/network.py", line 602, in get_all_interfaces_definition
     dhclient_info = get_dhclient_info()
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/dhclient.py", line 54, in get_dhclient_info
     "dhclient", proc_path=proc_path)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/ps.py", line 51, in get_running_pids_with_command
     os.path.join(proc_path, pid, "comm")).strip()
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/fs.py", line 347, in read_text_file
     return infile.read()
   File "/usr/lib/python3.5/codecs.py", line 698, in read
     return self.reader.read(size)
   File "/usr/lib/python3.5/codecs.py", line 493, in read
     newdata = self.stream.read()
 builtins.ProcessLookupError: [Errno 3] No such process