maint: Fix cert app to print help and exit if no subcommand

In 2.6.x this was the behavior, but the changes to the way options
parsing worked in 2.7.x to support faces changed the behavior of how
help was called.

This resulted in the follow unhelpful error message when you just called
puppet cert:

/Users/matthewrobinson/work/puppet/lib/puppet/ssl/certificate_authority/interface.rb:85:in `method='
Invalid method  to apply

Reviewed-by: Pieter van de Bruggen <pieter@puppetlabs.com>