sshutils: shutdown all connections before exiting

Register each SSHClient object's close() method with the atexit module.
This *should* close any open SSH connections on interpreter shutdown and
in theory prevent the dreaded "Exception in thread Thread-21 (most
likely raised during interpreter shutdown)" error with paramiko.

closes gh-135

starcluster/sshutils/__init__.py

@@ -3,6 +3,7 @@
 import sys
 import stat
 import glob
+import atexit
 import string
 import socket
 import fnmatch
@@ -61,6 +62,7 @@ def __init__(self,
             raise exception.SSHNoCredentialsError()
         self._glob = SSHGlob(self)
         self.__last_status = None
+        atexit.register(self.close)
 
     def load_private_key(self, private_key, private_key_pass=None):
         # Use Private Key.