webadmin: Expected data type fixed in AsyncDataProvider

Due to change in in backend, LinkedList is returned instead of expected
ArrayList that leads to ClassCactException.

Fix: General interface List is now expected.

Change-Id: I42199b7ac32f68c7db0d74fa48774d065806b37b
Signed-off-by: Jakub Niedermertl <jniederm@redhat.com>