frontend: fix getHostListByDataCenter

ClassCastException was thrown in
AsyncDataProvider#getHostListByDataCenter claiming that LinkedList
cannot be casted to ArrayList.

We should not assume the returned List is ArrayList. This patch fix it.

Change-Id: I14ac504114942785e512ab83e783f07ded69ca47
Signed-off-by: Arik Hadas <ahadas@redhat.com>