etcd: Bump to new 3.4.x version

This moves to the newest etcd release, and also updates the imports to
the new go.etcd.io path. I think this is a bit of a pain, but might as
well get it done.

etcd/methods.go

@@ -24,9 +24,9 @@ import (
 
 	"github.com/purpleidea/mgmt/util/errwrap"
 
-	etcd "github.com/coreos/etcd/clientv3"
-	etcdutil "github.com/coreos/etcd/clientv3/clientv3util"
-	etcdtypes "github.com/coreos/etcd/pkg/types"
+	etcd "go.etcd.io/etcd/clientv3"
+	etcdutil "go.etcd.io/etcd/clientv3/clientv3util"
+	etcdtypes "go.etcd.io/etcd/pkg/types"
 )
 
 // volunteer offers yourself up to be a server if needed. If you specify a nil