Add initial support for Hetzner Cloud

This is an initial attempt to make etcd-manager work with Hetzner Cloud.
Sample log:
volumes.go:56] Found ID of the running server: 19820911
volumes.go:73] Found name of the running server: "master-fsn1.masters.test1.k8s.local"
volumes.go:73] Found location of the running server: "fsn1"
volumes.go:78] Found first private net IP of the running server: "10.10.0.3"
main.go:317] Mounting available etcd volumes matching tags [KubernetesCluster=test1.k8s.local]; nameTag=etcdCluster-main
volumes.go:105] Finding attachable etcd volumes
volumes.go:129] Found attachable volume etcd-fsn1.test1.k8s.local(18745085) with status "available"
mounter.go:304] Trying to mount master volume: "18745085"
volumes.go:195] Attaching volume etcd-fsn1.test1.k8s.local(18745085) to the running server
volumes.go:189] Attached volume etcd-fsn1.test1.k8s.local(18745085) to the running server
mounter.go:318] Currently attached volumes: [0xc000445c80]
mounter.go:72] Master volume "18745085" is attached at "/dev/disk/by-id/scsi-0HC_Volume_18745085"
mounter.go:86] Doing safe-format-and-mount of /dev/disk/by-id/scsi-0HC_Volume_18745085 to /mnt/hcloud-18745085
volumes.go:156] Finding mounted volume "/dev/disk/by-id/scsi-0HC_Volume_18745085"
volumes.go:159] Found mounted volume "/dev/disk/by-id/scsi-0HC_Volume_18745085"
mounter.go:125] Found volume "18745085" mounted at device "/dev/disk/by-id/scsi-0HC_Volume_18745085"
mounter.go:171] Creating mount directory "/rootfs/mnt/hcloud-18745085"
mounter.go:176] Mounting device "/dev/disk/by-id/scsi-0HC_Volume_18745085" on "/mnt/hcloud-18745085"
mounter.go:224] mounting inside container: /rootfs/dev/disk/by-id/scsi-0HC_Volume_18745085 -> /rootfs/mnt/hcloud-18745085
mounter.go:94] mounted master volume "18745085" on /mnt/hcloud-18745085
volumes.go:215] Found first private IP of the running server: 10.10.0.3
main.go:332] discovered IP address: 10.10.0.3
main.go:337] Setting data dir to /rootfs/mnt/hcloud-18745085
server.go:87] starting GRPC server using TLS, ServerName="etcd-manager-server-vol-18745085"
main.go:485] peerClientIPs: [10.10.0.3]
etcdserver.go:560] starting etcd with state cluster:<cluster_token:"OTBR-ltoYKXszZC0Nf32yQ" nodes:<name:"vol-18745085" peer_urls:"https://vol-18745085.internal.test1.k8s.local:2380" client_urls:"https://vol-18745085.internal.test1.k8s.local:4001" quarantined_client_urls:"https://vol-18745085.internal.test1.k8s.local:3994" tls_enabled:true > > etcd_version:"3.5.3"
etcdserver.go:569] starting etcd with datadir /rootfs/mnt/hcloud-18745085/data/OTBR-ltoYKXszZC0Nf32yQ
server.go:105] GRPC server listening on "10.10.0.3:3996"
discovery.go:35] Discovering volumes for "etcdCluster-main"
discovery.go:59] Discovered volume etcd-fsn1.test1.k8s.local(18745085) attached to server master-fsn1.masters.test1.k8s.local(19820911)
peers.go:116] found new candidate peer from discovery: vol-18745085 [{10.10.0.3 0}]
peers.go:296] connecting to peer "vol-18745085" with TLS policy, servername="etcd-manager-server-vol-18745085"
hosts.go:84] hosts update: primary=map[], fallbacks=map[vol-18745085.internal.test1.k8s.local:[10.10.0.3]], final=map[10.10.0.3:[vol-18745085.internal.test1.k8s.local]]
/cc @justinsb @olemarkus
kubernetes-retired / etcdadm

Add initial support for Hetzner Cloud #309
