use neutron dhcp_domain in nova.conf (bnc#865733)

(cherry picked from commit ed372138b2d942f303d8da10e1121a594c2afaba)

Conflicts:
	chef/cookbooks/nova/recipes/config.rb
Merge pull request #334 from vuntz/release/stoney/master

use neutron dhcp_domain in nova.conf (bnc#865733)