webadmin: amend style for confirm host reboot dialog

To accommodate PatternFly changes, amend style for
'Confirm host has been rebooted' dialog
(avoid vertical scrollbar/missing padding).

Before-Screenshot: http://i.imgur.com/oEpNKkG.jpg
After-Screenshot: http://i.imgur.com/TEPzHFZ.jpg

Change-Id: I16f43d896014143b1eb90276d8b625b94589d7e6
Signed-off-by: Daniel Erez <derez@redhat.com>