webadmin: Validate no duplicate labels were input

It was previously possible to supply the same label twice in the
dialog, now this triggers a validation error.

Change-Id: Ia58f0a84c3ef27300f22b68ea6a405c609cf8815
Bug-Url: https://bugzilla.redhat.com/1077098
Signed-off-by: Lior Vernia <lvernia@redhat.com>