[CHEF-5057] Add default choice option to confirm prompt

This commit allows a default choice to be provided for a ui.confirm
prompt.  "-y" flag still takes precedence, and the "default" for the
default is the previous behavior ("Y/N", no default choice).

Testing done:

bundle exec rake spec - passes
bundle exec rspec spec/unit/knife/core - passes
Remove should_exit out of parameters and make ui.confirm less ugly.
Contributions infor for CHEF-5057.