Crash when trying to find matching scheme
From the CMS instance, I noticed this exception thrown in the `conveyor-transfer-submitter` after creating some minimal RSEs and trying to trigger a transfer
```
2018-01-11 20:59:36,983	4217	CRITICAL	Exception happened when trying to get transfer for request 8a818df9ca4047e3a6b36f95b98a5c7e: Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/rucio/core/transfer.py", line 567, in get_transfer_requests_and_source_replicas
    scheme=current_schemes)
  File "/usr/lib/python2.7/site-packages/rucio/rse/rsemanager.py", line 640, in find_matching_scheme
    dest_candidates = sorted(dest_candidates, key=lambda k: k['domains'][domain][operation_dest])
  File "/usr/lib/python2.7/site-packages/rucio/rse/rsemanager.py", line 640, in <lambda>
    dest_candidates = sorted(dest_candidates, key=lambda k: k['domains'][domain][operation_dest])
KeyError: None
```

diff --git a/lib/rucio/core/transfer.py b/lib/rucio/core/transfer.py
index c3ae3e0f9..35b6cd1e6 100644
--- a/lib/rucio/core/transfer.py
+++ b/lib/rucio/core/transfer.py
@@ -7,7 +7,7 @@
 #
 # Authors:
 # - Martin Barisits, <martin.barisits@cern.ch>, 2017
-# - Mario Lassnig, <mario.lassnig@cern.ch>, 2017
+# - Mario Lassnig, <mario.lassnig@cern.ch>, 2017-2018
 
 import datetime
 import json
@@ -558,12 +558,13 @@ def get_transfer_requests_and_source_replicas(process=None, total_processes=None
                 allow_tape_source = True
 
                 # Find matching scheme between destination and source
+                # TODO: In the future, move to domain "third_party_copy"
                 try:
                     matching_scheme = rsemgr.find_matching_scheme(rse_settings_dest=rses_info[dest_rse_id],
                                                                   rse_settings_src=rses_info[source_rse_id],
                                                                   operation_src='read',
                                                                   operation_dest='write',
-                                                                  domain=None,
+                                                                  domain='wan',
                                                                   scheme=current_schemes)
                 except RSEProtocolNotSupported:
                     logging.error('Operation "write" not supported by %s with schemes %s' % (rses_info[dest_rse_id]['rse'], current_schemes))
['lib/rucio/core/transfer.py']