Fix an exexption about AssertionError

Previously throw an exception about "Miss keys:set(['snmp.
password_confirmation'])" AssertionError when configuring
SNMP password
Now fix it.

Change-Id: Ifc449bbb623e9f7407d32be5888db9e7d335abe2
Bug-Url:https://bugzilla.redhat.com/show_bug.cgi?id=984458
Signed-off-by: hadong <hadong0720@gmail.com>