webadmin: Make reinstall dialog slightly bigger

This to obviate a scroll bar that appears when selecting a provider.

Change-Id: Icdaba03bcefb5f65ca4bdecb7008b48c7a0758d1
Bug-Url: https://bugzilla.redhat.com/1117399
Signed-off-by: Lior Vernia <lvernia@redhat.com>