restapi: added support for optionally select the target cluster

An optional parameter cluste.id has been added to the migrate.

Change-Id: I66aa4041b5308fe4f7f1038f53715cd78c5c279f
Bug-Url: https://bugzilla.redhat.com/1150191
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>