Add config parameter 'live_migration_scheme' to live migration with tls guide

This patch adds the config option 'live_migration_scheme = tls' to the
secure live migration guide.

To let the live migration use the qemu native tls, some configuration of
the compute nodes is needed. The guide describes this but misses the
'live_migration_scheme' config option.

It is necessary to set 'live_migration_scheme' to tls to use the
connection uri for encrypted traffic. Without this parameter everything
seems to work, but the unencrypted tcp-connection is still used for the
live migration.

Closes-Bug: #1919357
Change-Id: Ia5130d411706bf7e1c983156158011a3bc6d5cd6
(cherry picked from commit 5d5ff82)

Author: josephineSei

doc/source/admin/secure-live-migration-with-qemu-native-tls.rst

@@ -120,17 +120,26 @@ Performing the migration
 
 (1) On all relevant compute nodes, enable the
     :oslo.config:option:`libvirt.live_migration_with_native_tls`
-    configuration attribute::
+    configuration attribute and set the
+    :oslo.config:option:`libvirt.live_migration_scheme`
+    configuration attribute to tls::
 
        [libvirt]
        live_migration_with_native_tls = true
+       live_migration_scheme = tls
 
     .. note::
         Setting both
         :oslo.config:option:`libvirt.live_migration_with_native_tls` and
         :oslo.config:option:`libvirt.live_migration_tunnelled` at the
         same time is invalid (and disallowed).
 
+    .. note::
+        Not setting
+        :oslo.config:option:`libvirt.live_migration_scheme` to ``tls``
+        will result in libvirt using the unencrypted TCP connection
+        without displaying any error or a warning in the logs.
+
     And restart the ``nova-compute`` service::
 
         $ systemctl restart openstack-nova-compute