Adds scenario for DNS-nameserver configuration

Tests that subnet's DNS server configurations are effecting the VMs
1. Configure subnet with DNS nameserver
2. Retrieve the VM's configured DNS and verify it matches the one configured
   for the subnet.
3. Update subnet's DNS
4. Retrieve the VM's configured DNS and verify it matches the new one
   configured for the subnet.

DNS nameservers' addresses are arbitrary as any resolution check would be
testing either:
* L3 forwarding - tested in test_network_basic_ops
* External DNS service resolution - out of scope for Tempest tests

Adds dhcp_client to tempest.conf which should indicate the image DHCP client
used for renewing dhcp lease.
Supported: 'udhcpc', 'dhclient'
Empty value should skip subnet update parts of scenario

TODO: add support for dhcpcd client

Change-Id: Idd8d11929320aa9208dd14d718e1f8f264d9de80
6 files changed