Set NeutronDnsDomain for uni01alpha DNS tests

Set it to openstackgate.local to match the neutron tempest plugin
default.

Author: renjingxiao

scenarios/uni01alpha/config_download.yaml

@@ -48,6 +48,7 @@ parameter_defaults:
   ComputeExtraConfig:
     nova::compute::libvirt::services::libvirt_virt_type: qemu
     nova::compute::libvirt::virt_type: qemu
+  NeutronDnsDomain: 'openstackgate.local'
   BarbicanSimpleCryptoGlobalDefault: true
   Debug: true
   DockerPuppetDebug: true