add default values for configuration

Remove possible error when upgrading Katello. Scenario:
1.   new option is introduced to katello.yml
2.   user updates `yum update` Katello
3.   configfile `/etc/katello/katello.yml` does not have the new option, Katello will refuse to start

Fixed by introducing default values to `config/katello_defaults.yml`. The new option is added there.