Don't output auth tokens with trace output

The tokens are huge and make it difficult to follow the calls and are not
usually of interest.

Change-Id: Id35eeabedd7b460bf4d200770b2412b5233aed45
diff --git a/tempest/common/rest_client.py b/tempest/common/rest_client.py
index 830968e..43907d9 100644
--- a/tempest/common/rest_client.py
+++ b/tempest/common/rest_client.py
@@ -282,7 +282,7 @@
         return ""
 
     def _log_request(self, method, req_url, resp,
-                     secs="", req_headers=None,
+                     secs="", req_headers={},
                      req_body=None, resp_body=None):
         # if we have the request id, put it in the right part of the log
         extra = dict(request_id=self._get_request_id(resp))
@@ -306,6 +306,8 @@
         # world this is important to match
         trace_regex = CONF.debug.trace_requests
         if trace_regex and re.search(trace_regex, caller_name):
+            if 'X-Auth-Token' in req_headers:
+                req_headers['X-Auth-Token'] = '<omitted>'
             log_fmt = """Request (%s): %s %s %s%s
     Request - Headers: %s
         Body: %s