Migrate k8s data to etcd3 api store

Default backend is now etcd3 (was etcd2).
The migration process consists of the following steps:
* check if migration is necessary
* stop etcd on first etcd server
* run migration script
* start etcd on first etcd server
* stop kube-apiserver until configuration is updated
* update kube-apiserver
* purge old etcdv2 data