ipautil: remove get_domain_name()
get_domain_name() and related code depends on ipaplatform.
Replace all uses of get_domain_name() with api.env.domain and remove
get_domain_name() and all of the related code.
https://fedorahosted.org/freeipa/ticket/6474
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>