initialize: use correct heuristic to check if etcdenvironment is set

In some circumstances (e.g. nova-agent-watcher) cloudconfig files will
be created where the EtcdEnvironment is an empty map, and hence != nil.
If this is the case we should not do anything at all (because the user
hasn't explicitly asked us to configure etcd). This change standardises
behaviour with the check that we already do for FleetEnvironment.
Merge pull request #200 from jonboulle/fu

initialize: use correct heuristic to check if etcdenvironment is set