Do not modify in place a string which is an attribute of the node

Especially as it's used for naming a service resource. The result is
that we get:

service: no such service neutron-linuxbridge-agent
I guess I find that also unreadable (and we do want to match from the beginning). How about

```ruby
  neutron_agent = neutron_agent[/openstack-(.*)/, 1]
```

BTW, I think we have the agent name already somewhere in the attributes, we might just use that one instead.