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

doc/api_samples/versions/versions-get-resp.json

@@ -22,7 +22,7 @@
                 }
             ],
             "status": "CURRENT",
-            "version": "2.55",
+            "version": "2.56",
             "min_version": "2.1",
             "updated": "2013-07-23T11:33:21Z"
         }