Fix crash on deactivating the ceph proposal

```
F, [2014-07-18T10:02:35.431448 #27874] FATAL -- : apply_role: Exception undefined method `+' for nil:NilClass /opt/dell/crowbar_framework/app/models/node_object.rb:87:in `find_node_by_name'
/opt/dell/crowbar_framework/app/models/ceph_service.rb:109:in `apply_role_pre_chef_call'
/opt/dell/crowbar_framework/app/models/service_object.rb:1212:in `apply_role'
/opt/dell/crowbar_framework/app/models/service_object.rb:569:in `destroy_active'
/opt/dell/crowbar_framework/app/controllers/barclamp_controller.rb:415:in `proposal_update'
```