move node_to_str() and str_to_node() from sheep/cluster.h to include/internal_proto.h

shephered uses node_to_str() and str_to_node(), so they should be
moved to the header which can be included by other directories of
sheep/.
move enum cluster_join_result from sheep/cluster.h to include/internal_proto.h

enum cluster_join_result is also used by shepherd. So this patch
moves it from sheep/ to include/.
shepherd: a new cluster manager specialized for sheepdog

This patch adds shepherd, a new cluster manager for sheepdog.

shepherd cluster manager is designed as master-slave. So it can be
SPOF. But I'm planning to let it be redundant by utilizing corosync.
If it can be improved, this can be an alternative of the group
management part of ZooKeeper, and it doesn't depend on JVM.

This patch adds the new directory shepherd/ for it, and the
executable binary will be produced as shepherd/shepherd.
The cluster driver for sheep is added as sheep/cluster/shepherd.c.

If you have some opinions or feature requests for a cluster manager
designed and implemented from scratch, I'd like to hear your comments.