Include custom roles in the Default User Role dropdown for new user auto-creation

[arobertn]

When configuring for automatic user creation there is the option to select the role the new user will be given. Unfortunately the only options here are the graylog built-in roles Admin and Reader. It would be nice if you could select other roles that you have created in the system. In this case, the Reader role should be combined with the custom role selected.

[jalogisch]

this is related to Graylog2/graylog2-server#3968 and if that will be possible it should be no issue to implement.