ovn-controller: Add support for load balancing.

ovn-controller: Add support for load balancing.

ovn-controller now supports 2 new logical actions.

1. ct_lb;
Sends the packet through the conntrack zone to NAT
packets. Packets that are part of established connection
will automatically get NATed based on the NAT arguments
supplied to conntrack when the first packet was committed.

2. ct_lb(192.168.1.2, 192.168.1.3);
   ct_lb(192.168.1.2:80, 192.168.1.3:80);
Creates an OpenFlow group with multiple buckets and equal weights
that changes the destination IP address (and port number) of the packet
statefully to one of the options provided inside the parenthesis.

Signed-off-by: Gurucharan Shetty <guru@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>