change for loop to use new API

ca_shell.py

@@ -140,7 +140,7 @@ class CAManagerShell(cmd.Cmd, object):
 
 
 def print_available_authorities(ca_manager):
-    for i, ca_item  in enumerate(ca_manager.get_cas_list()):
+    for i, ca_item  in enumerate(ca_manager.ca):
         (ca_id, ca_name, ca_type) = ca_item
         print("- %d : [%3s] %-15s (%s)" % (i ,ca_type, ca_id, ca_name))