Improve logging of credentials

Omit v3 tokens from response logging. Implement credentials logging
for TestResources objects.

Change-Id: I213585e691f58e380a4be463b493591f1449422e
diff --git a/tempest/common/cred_provider.py b/tempest/common/cred_provider.py
index a4b2ae8..5cce0bb 100644
--- a/tempest/common/cred_provider.py
+++ b/tempest/common/cred_provider.py
@@ -88,6 +88,11 @@
     def __getattr__(self, item):
         return getattr(self._credentials, item)
 
+    def __str__(self):
+        _format = "Credentials: %s, Network: %s, Subnet: %s, Router: %s"
+        return _format % (self._credentials, self.network, self.subnet,
+                          self.router)
+
     def set_resources(self, **kwargs):
         for key in kwargs.keys():
             if hasattr(self, key):