webadmin: Add new role permission drop below the end of the screen

When creating a new role, the permission tree ends
below the lower border of the screen when the
entire tree is expanded

Change-Id: I6f785d91c8bc10b0ceb44ed153237e5f62590b76
Bug-Url: https://bugzilla.redhat.com/1057103
Signed-off-by: Ravi Nori <rnori@redhat.com>