Simplify code by using mask_dict_password (again)

A patch to oslo.utils was done to make mask_dict_password() work like
mask_password(). Using mask_dict_password() makes the code simpler and
easier to understand.

There was a previous patch (6ad85298c4)
similar to this but it missed changing this occurrence.

Change-Id: I25233e49555cee9f856c755b63234cb7ab31d6aa

ironic/drivers/modules/agent_base_vendor.py

@@ -16,7 +16,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import ast
 import collections
 import time
 
@@ -787,8 +786,8 @@ class BaseAgentVendor(AgentDeployMixin, base.VendorInterface):
 
         ndict = node.as_dict()
         if not context.show_password:
-            ndict['driver_info'] = ast.literal_eval(
-                strutils.mask_password(ndict['driver_info'], "******"))
+            ndict['driver_info'] = strutils.mask_dict_password(
+                ndict['driver_info'], "******")
 
         return {
             # heartbeat_timeout is a config, so moving it into the