ovn: Add support for DHCP option 15 - domain name

ovn: Add support for DHCP option 15 - domain name

For Openstack Internal DNS functionality we need
to provide support for domain_name option.
DHCP option 15 was previously used only in parser
tests and according to RFC it should be renamed to
domain_name [1].

This patch modifies its name in the tests from
'domain' to 'domain_name' and adds its support
to the code.

[1] https://tools.ietf.org/html/rfc2132#section-3.17

Signed-off-by: Maciej Józefczyk <mjozefcz@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>