add strategy host_maintenance

maintain one compute node without having the user's application
been interruptted.
It will firstly migrate all instances from the maintenance node
to one backup node. If not, it will migrate all instances,
relying on nova-schduler.

Change-Id: I29ecb65745d5e6ecab41508e9a91b29b39a3f0a8
Implements:blueprint cluster-maintaining

watcher/decision_engine/goal/goals.py

@@ -241,3 +241,28 @@ class HardwareMaintenance(base.Goal):
     def get_efficacy_specification(cls):
         """The efficacy spec for the current goal"""
         return specs.HardwareMaintenance()
+
+
+class ClusterMaintaining(base.Goal):
+    """ClusterMaintenance
+
+    This goal is used to maintain compute nodes
+    without having the user's application being interrupted.
+    """
+
+    @classmethod
+    def get_name(cls):
+        return "cluster_maintaining"
+
+    @classmethod
+    def get_display_name(cls):
+        return _("Cluster Maintaining")
+
+    @classmethod
+    def get_translatable_display_name(cls):
+        return "Cluster Maintaining"
+
+    @classmethod
+    def get_efficacy_specification(cls):
+        """The efficacy spec for the current goal"""
+        return specs.Unclassified()