bridge: Allow users to configure statistics update to OVSDB.

This commit adds a new configuration "stats-update-interval" in
"other_config" of Open_Vswitch table.  So users can control the
statistics update frequency.  A possible use case is that, users
can lower the update frequency to reduce the cpu consumption of
the ovs-vswitchd thread.

The configured value should always be greater than or equal to
5000 ms.  And more frequent statistics update should be achieved
via OpenFlow.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>