Enable cold migration with target host(2/2)

This function enables users to specify a target host
when cold migrating a VM instance.

This patch modifies the migration API.

APIImpact
    Add an optional parameter 'host' in cold migration action.

Change-Id: Iee356c4dd097c846b6ca8617ead6a061300c83f8
Implements: blueprint cold-migration-with-target-queens

api-ref/source/servers-admin-action.inc

@@ -56,10 +56,15 @@ Migrate Server (migrate Action)
 
 .. rest_method:: POST /servers/{server_id}/action
 
-Migrates a server to a host. The scheduler chooses the host.
+Migrates a server to a host.
 
 Specify the ``migrate`` action in the request body.
 
+Up to microversion 2.55, the scheduler chooses the host.
+Starting from microversion 2.56, the ``host`` parameter is available
+to specify the destination host. If you specify ``null`` or don't specify
+this parameter, the scheduler chooses a host.
+
 Policy defaults enable only users with the administrative role to
 perform this operation. Cloud providers can change these permissions
 through the ``policy.json`` file.
@@ -76,12 +81,18 @@ Request
 
   - server_id: server_id_path
   - migrate: migrate
+  - host: host_migration_2_56
 
-**Example Migrate Server (migrate Action)**
+**Example Migrate Server (migrate Action) (v2.1)**
 
 .. literalinclude:: ../../doc/api_samples/os-migrate-server/migrate-server.json
    :language: javascript
 
+**Example Migrate Server (migrate Action) (v2.56)**
+
+.. literalinclude:: ../../doc/api_samples/os-migrate-server/v2.56/migrate-server.json
+   :language: javascript
+
 Response
 --------