This patch adds the dhcp_domain config entry to nova.conf and
implements group_vars to default both nova and neutron's dhcp
domain values to be the same.
The individual values can still be overridden in user_variables
by using nova_dhcp_domain or neutron_dhcp_domain, but it's expected
that deployers would like these to be consistent.
Change-Id: I97beb78f62aeca2665ff72805056d36ead2adaaa
Closes-Bug: #1482045
7427775201