add no-default-user option to active_admin:devise generator

When using active admin with a customized User model living in the
main app, it is common to add further validations for profile
attributes etc. Creating a user in the generated migration may fail
then since not all required attributes are present.

In those cases it's better to setup the initial user via a database
seed. This option allows other eniges, which bundle active admin, to
invoke default active admin generators while still providing their
default user via a seed.