Use assertTrue and assertFalse vs assertEqual

Using assertTrue and assertFalse is more readable while
checking a boolean result.

Note: In the following files we have to use assertEqual,
      since the api returns the value as unicode:
tempest/api/volume/test_volumes_get.py
65:            self.assertEqual('true', fetched_volume['bootable'])
65:            self.assertEqual('true', fetched_volume['bootable'])
114:            self.assertEqual('true', updated_volume['bootable'])
114:            self.assertEqual('true', updated_volume['bootable'])
tempest/api/volume/test_volumes_list.py
188:            self.assertEqual('false', volume['bootable'])
tempest/api/volume/test_volumes_get.py
67:            self.assertEqual('false', fetched_volume['bootable'])
116:            self.assertEqual('false', updated_volume['bootable'])
tempest/api/compute/servers/test_servers_negative.py
533:        self.assertEqual('false', nonbootable_vol['bootable'])

Change-Id: I004a7860847e8361280d3fa105d2dfa97d99bee4
diff --git a/tempest/api/volume/test_volumes_backup.py b/tempest/api/volume/test_volumes_backup.py
index 552b231..07cfad5 100644
--- a/tempest/api/volume/test_volumes_backup.py
+++ b/tempest/api/volume/test_volumes_backup.py
@@ -128,7 +128,7 @@
 
         volume_details = self.volumes_client.show_volume(
             volume['id'])['volume']
-        self.assertEqual('true', volume_details['bootable'])
+        self.assertTrue(volume_details['bootable'])
 
         # Create a backup
         backup = self.create_backup(volume_id=volume['id'])
@@ -140,7 +140,7 @@
         restored_volume_info = self.volumes_client.show_volume(
             restored_volume_id)['volume']
 
-        self.assertEqual('true', restored_volume_info['bootable'])
+        self.assertTrue(restored_volume_info['bootable'])
 
 
 class VolumesBackupsV39Test(base.BaseVolumeTest):