HBASE-12680 Move signal related code from ClusterManager to HBaseClusterManager and change ClusterManager to an interface

Summary: The reason of this change is to make us write implementation of ClusterManager not using ssh/unix signals.

Test Plan: The compilation is OK.

Reviewers: eclark, manukranthk

Differential Revision: https://reviews.facebook.net/D30201

Signed-off-by: Elliott Clark <eclark@apache.org>