prevent crash when removing nova-compute role from a node

This crash, that was fixed in the [last commit](https://github.com/crowbar/barclamp-ceilometer/commit/d0aaa0347af276d74b407f3428fca52387c9ee03#diff-d41d8cd98f00b204e9800998ecf8427e) remains when you remove the `nova-multi-compute` role from a node, probably because of the nova attributes still in the node object. until the proper role deactivation is not yet implemented this needs some workaround.