Adding SNMPd auth credentials

-Undercloud Ceilometer has to have access to SNMPd credentials,
 so it can poll the Overcloud nodes
-In every Overcloud node, we need to set the same cretentials
 to SNMPd.conf

Change-Id: Icf7c0c1772b6380b7136108e61c15cafe17274ba
Merge "Adding SNMPd auth credentials"