Change assertTrue(isinstance()) by optimal assert

Some of tests use different method of assertTrue(isinstance(A, B)) or
assertEqual(type(A), B). The correct way is to use assertIsInstance(A,
B) provided by testtools.

Change-Id: Ie98c1aec65281fa1784070ce76e472caca7cfc47
Closes-bug: #1268480
diff --git a/tempest/api/baremetal/admin/test_nodes.py b/tempest/api/baremetal/admin/test_nodes.py
index 8cc4d05..2c44665 100644
--- a/tempest/api/baremetal/admin/test_nodes.py
+++ b/tempest/api/baremetal/admin/test_nodes.py
@@ -138,14 +138,14 @@
         body = self.client.get_node_boot_device(self.node['uuid'])
         self.assertIn('boot_device', body)
         self.assertIn('persistent', body)
-        self.assertTrue(isinstance(body['boot_device'], six.string_types))
-        self.assertTrue(isinstance(body['persistent'], bool))
+        self.assertIsInstance(body['boot_device'], six.string_types)
+        self.assertIsInstance(body['persistent'], bool)
 
     @test.idempotent_id('3622bc6f-3589-4bc2-89f3-50419c66b133')
     def test_get_node_supported_boot_devices(self):
         body = self.client.get_node_supported_boot_devices(self.node['uuid'])
         self.assertIn('supported_boot_devices', body)
-        self.assertTrue(isinstance(body['supported_boot_devices'], list))
+        self.assertIsInstance(body['supported_boot_devices'], list)
 
     @test.idempotent_id('f63b6288-1137-4426-8cfe-0d5b7eb87c06')
     def test_get_console(self):