ovn-nb: Change how router ports work.

ovn-nb: Change how router ports work.

This is for two reasons.  First, a router port is not really much of a
special case from a logical switch's point of view.  For switching
purposes, it works exactly the same as any other port.  Having a special
column for it just adds artificial special cases.

Second, the previous form of router ports specified that all of them use
the logical port name "ROUTER".  This seemed to make sense at the time but
now it is just adding more special cases.  Instead just giving them names
like any other port makes life easier.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>