cfgupgrade: Add confirmation message

A message will be given instead of just dropping the user back to the
prompt in case of a successful upgrade.

[…]
documentation formats). Continue with upgrading configuration?
y/[n]/?: y
Configuration successfully upgraded for version 2.5.0~beta1.
root@master:~#

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>