koji-ssl-admin: add uid attribute to user certificates
Generate user certificates with a "UID" attribute in the subject field.
For now Koji's instructions tell users to configure
"DNUsernameComponent = CN". Eventually we may want to switch "CN" over
to "UID". Some certificate authorities do not set the "CN" attribute to
a simple username. For example Red Hat Certificate Server sets "CN" to
"Ken Dreyer kdreyer", and "UID" to "kdreyer".
As a first step, set both "CN" and "UID" attributes to the same value.