ovn-controller: Explicitly pass the flow table from function to function.

ovn-controller: Explicitly pass the flow table from function to function.

As I was working in ovn-controller, I found it hard to tell what code
produced and what code consumed the OpenFlow flow table, because it was
all implicit.  This commit makes the data structure an explicit variable
in the main loop, which makes it easier to see.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Russell Bryant <rbryant@redhat.com>