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.

This should fix coreos/bugs#100

/cc @crawford