Enable dns_domain_ports extension in tempest config

This patch adds missing dns-domain-ports extension to tempest
config to not skip related tests.

It also fixes small issue with getting port's dns_domain attribute
from update port response's body.

Change-Id: I2d50039f864f9beb7f7f9f76c7655a9bb05e9df4
Closes-Bug: #1762369
2 files changed