Nodestate API test for power state set

This patch implements Nodestate API tests, to set the power state of
the node for the following power states -
1. power on
2. rebooting
3. power off

Change-Id: Icbb2719ffae772878f183c0c77c1e9d9028655c1
diff --git a/tempest/services/baremetal/v1/base_v1.py b/tempest/services/baremetal/v1/base_v1.py
index 296a199..52479b5 100644
--- a/tempest/services/baremetal/v1/base_v1.py
+++ b/tempest/services/baremetal/v1/base_v1.py
@@ -226,3 +226,16 @@
         """
 
         return self._patch_request('ports', uuid, patch)
+
+    @base.handle_errors
+    def set_node_power_state(self, node_uuid, state):
+        """
+        Set power state of the specified node.
+
+        :param node_uuid: The unique identifier of the node.
+        :state: desired state to set (on/off/reboot).
+
+        """
+        target = {'target': state}
+        return self._put_request('nodes/%s/states/power' % node_uuid,
+                                 target)