Comment 6 for bug 1774052

Reviewed: https://review.openstack.org/571679
Committed: https://git.openstack.org/cgit/openstack/networking-ovn/commit/?id=c3a383e9f7727ee3e40b1c2045dc75f3beb0d7d8
Submitter: Zuul
Branch: stable/queens

commit c3a383e9f7727ee3e40b1c2045dc75f3beb0d7d8
Author: Numan Siddique <email address hidden>
Date: Wed May 30 00:49:57 2018 +0530

    Add default dns_server option in DHCP_Options table

    If a subnet is not defined with dns_nameservers option, then networking-ovn
    doesn't add any default 'dns_server' DHCPv4 option in the DHCP_Options table.

    This patch addresses this issue by
     - Introducing a config option - 'dns_servers' similar to 'dnsmasq_dns_servers'
       config option supported by dhcp agent and uses it as the 'dns_server' DHCPv4 option
       in the DHCP_Options table.

     - If the 'dns_servers' option is not defined, then it uses the system dns resolvers
       from the /etc/resolv.conf.

    Closes-bug: #1774052
    Change-Id: Ibf6396232f8eac780baa676b0b8395082d5c3527
    (cherry picked from commit ff5a4a0f6b8867066b666d5e63116691e8d223bb)