add string representation for context

in the cases where context manages to make it's way into a log message
we have no sane representation of it. The to_dict method provides the
relevant information, so we can use a dump of that as our human
friendly version.

Change-Id: I394260c6957ba5c4f2b134e75cf8e7bd1d1b59c2

nova/context.py

@@ -188,6 +188,9 @@ class RequestContext(context.RequestContext):
 
         return context
 
+    def __str__(self):
+        return "<Context %s>" % self.to_dict()
+
 
 def get_admin_context(read_deleted="no"):
     return RequestContext(user_id=None,