EtcdOperationActor improvements

[x] EtcdOperationActor should not attempt to handle server reply timeout, this needs to be handled in EtcdClient itself (#6)
[x] unlimited number of retries should be supported (in some cases the program cannot proceed until certain etcd operation is successfully executed)
[x] EtcdClient failures should be handled
[x] tests are missing

rkrzewski / akka-cluster-etcd