webadmin: allow migration to different cluster

Added a list box into the advanced parameters which lets the user change the
destination cluster. Since this is quite a dangerous task, added also a
warning.

Change-Id: I224ac719d16b348bb8ea31b256fe459bc372a88d
Bug-Url: https://bugzilla.redhat.com/1150191
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>