ipaserver: Fix DNS installation forward policy and DNSSEC validation

forward_policy needs to be None for the DNS check for proper initialization
if the user is not providing another forward_policy value. forward_policy will
be set in the DNS check.

no_dnssec_validation is enabled in the DNS check if the forwarders do not
provide DNSSEC validation. Therefore this needs to be handed over to the dns
installation later on.

New return values for forward_policy and no_dnssec_validation have been added
to the ipaserver_test module.