ipaserver_test: Do not use zone_overlap_check for domain name validation

The use of zone_overlay_check for the domain name validation is not good
for a repeated execution of the server deployment where setup_dns is
enabled. The zone overlay check will fail with "DNS zone X already exists
in DNS". zone_overlay_check is later on used in dns.install_check so it is
not needed to do it here also.

Fixes issues #164 (domain option validator should not call zone overlap..)