webadmin: Attach ISO dialog - adjusted dimensions

Adjusted dimensions of 'Attach ISO' dialog to prevent
help icon misalignment in French localization.

Change-Id: Id7a34a0b506b3c115413646905028a6c955292b3
Bug-Url: https://bugzilla.redhat.com/1001557
Signed-off-by: Daniel Erez <derez@redhat.com>