core: enable migration to a different cluster

Added a possibility to define the target cluster for the migration.

Change-Id: I9eb4fd7f10d3b0e63455a81f0b5c1389586a43f4
Bug-Url: https://bugzilla.redhat.com/1150191
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>