webadmin: escape during dialog loading causes problems.

- Added prevent default to escape handler to stop bad things
from happening when pressing escape while the dialog hasn't
fully loaded yet.

Change-Id: Ib6a217efa88a9fb3921aa12fcc86627cd005df1d
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=879497
Signed-off-by: Alexander Wels <awels@redhat.com>