server install: fix external CA install
Replace the dual definitions of domain_name, dm_password and admin_password
knobs in server install with single definitions using the original names
without the 'new_' prefix.
This fixes the options read from the installer option cache in step 2 of
external CA install to use the correct knob names.
https://fedorahosted.org/freeipa/ticket/6392
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>