delete_on_termination added for VM with cinder volume in 2 tests

Two tests in ServerBootFromVolumeStableRescueTest class don't clean
up volumes after finish. Adding delete_on_termination in block device
mappings (BDM) will clean created BDM cinder volumes when VM is
deleted.

Closes-Bug: #2114714
Change-Id: I4003a5f1e500f92106e1b1d2a0f4a7e9f0047f5d
diff --git a/tempest/api/compute/servers/test_server_rescue.py b/tempest/api/compute/servers/test_server_rescue.py
index d6c0324..9f96385 100644
--- a/tempest/api/compute/servers/test_server_rescue.py
+++ b/tempest/api/compute/servers/test_server_rescue.py
@@ -282,6 +282,7 @@
         """
         block_device_mapping_v2 = [{
             "boot_index": "0",
+            "delete_on_termination": "true",
             "source_type": "blank",
             "volume_size": CONF.volume.volume_size,
             "destination_type": "volume"}]
@@ -300,6 +301,7 @@
         """
         block_device_mapping_v2 = [{
             "boot_index": "0",
+            "delete_on_termination": "true",
             "source_type": "image",
             "volume_size": CONF.volume.volume_size,
             "uuid": CONF.compute.image_ref,