Create buttons later in ConfirmedDialog so users can define their own

Move the call to Super() a bit lower so user-defined buttons work.
Previously, the only button added was "Close". Also, we should
default to "OK" and "Cancel" since "Close" doesn't make much sense
in this context.

Change-Id: Id648b18996b75ea335bf3e3ce16da547663c6c68
Signed-off-by: Ryan Barry <rbarry@redhat.com>