Adding a DNS Server Config on the LS

[vamerick]

When inspecting flows in SB DB for DNS, they appear as follows:

  table=28(ls_in_dns_response ), priority=100  , match=(udp.dst == 53 && reg0[4]), action=(eth.dst <-> eth.src; ip6.src <-> ip6.dst; udp.dst = udp.src; udp.src = 53; outport = inport; flags.loopback = 1; output;)

The challenge is this is intercepting all udp port 53 packets. This PR is experimental, and is adding a DNS server configuration for logical switches. This server is added as a parameter in the flow rules. The new flow rules are supposed to validate udp.dst==53 and the destination ipv4 before attempting to check OVN for the DNS entries.

Built as follows:

cd ovs
./boot.sh
./configure
make -j$(nproc)
cd ..

./boot.sh
./configure --with-ovs-source=./ovs
make -j$(nproc)

Then tested as follows:

ubuntu@txdr-disk-bandwidth:~/ovn/ovn$ make check TESTSUITEFLAGS='-k "dns lookup"'
...
182: dns lookup : 1 HV, 2 LS, 2 LSPs/LS -- parallelization=yes -- ovn_monitor_all=yes ok
183: dns lookup : 1 HV, 2 LS, 2 LSPs/LS -- parallelization=yes -- ovn_monitor_all=no ok
184: dns lookup : dns_server_ip -- parallelization=yes -- ovn_monitor_all=yes ok
185: dns lookup : dns_server_ip -- parallelization=yes -- ovn_monitor_all=no ok
186: dns lookup : EDNS -- parallelization=yes -- ovn_monitor_all=yes skipped (ovn.at:12647)
187: dns lookup : EDNS -- parallelization=yes -- ovn_monitor_all=no skipped (ovn.at:12647)

Note
This requires additional testing. It is just experimental.