webadmin: Interface dialog- when opening the labels scroll should be down

when opening the network labels window, the scrollbar should be down,
so you should be able to click on '+' button without using scrollbar.

Being there fixed all the other classes that extends AddRemoveRowWidget
a have a scrollPanel.

Change-Id: I4a2d5bd7280ef98a6ac0e94c336fcc6f30666c8a
Bug-Url: https://bugzilla.redhat.com/1091695
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>