EventService exception handling to not log errors

In order to not log a StackNotFound exception as an error, the
identify_stack() method is now decorated with the
heat.openstack.common.rpc.client_exceptions wrapper.

Change-Id: I51adcfe4d86d312c9887e51e6c113f99e0f1b783
Closes-Bug: #1231135
Merge "EventService exception handling to not log errors"