greendns: Patch ssl

Newer (2.0+) dnspython imports ssl (and requests, which would have
eventually imported ssl), so greendns needs a monkey-patched ssl, too.
This should prevent a RecursionError with SSLContext.

Note that this *does not* bring full dnspython>=2.0 support, but our
version pin hasn't stopped people from trying to run with newer
dnspython anyway. May as well make it hurt a little less.

Fixes #677, but see also #619.

eventlet/support/greendns.py

@@ -42,6 +42,7 @@
 from eventlet.green import os
 from eventlet.green import time
 from eventlet.green import select
+from eventlet.green import ssl
 import six
 
 
@@ -55,6 +56,7 @@ def import_patched(module_name):
         'time': time,
         'os': os,
         'socket': _socket_nodns,
+        'ssl': ssl,
     }
     return patcher.import_patched(module_name, **modules)