Overcome "Connection Refused" on some operations

So some operations on Windows actually cause the WinRM service to become
unavailable for some time and WinRM currently cannot overcome this
problem. Since this is a real problem to us when e.g. installing SCVMM
on servers and we cannot influence the installer, here is the
work-around we implemented.

The current implementation simply tries 5 times with a 5 seconds delay,
but YMMV. Ideally this is configurable by the user, and future defaults
should be tested.

winrm/transport.py

@@ -1,8 +1,10 @@
 from __future__ import unicode_literals
 from contextlib import contextmanager
+import errno
 import re
 import sys
 import os
+import time
 import weakref
 
 is_py2 = sys.version[0] == '2'
@@ -189,7 +191,17 @@ def send_message(self, message):
         prepared_request = self.session.prepare_request(request)
 
         try:
-            response = self.session.send(prepared_request, timeout=self.read_timeout_sec)
+
+            # Retry connection on 'Connection refused'
+            for attempt in range(5):
+                try:
+                    response = self.session.send(prepared_request, timeout=self.read_timeout_sec)
+                    break
+                except requests.exceptions.ConnectionError as e:
+                    if attempt == 4 or 'connection refused' not in str(e).lower():
+                        raise
+                    time.sleep(5)
+
             response_text = response.text
             response.raise_for_status()
             return response_text