ovn-northd: Add support for static_routes.

ovn-northd: Add support for static_routes.

Logical patch ports are used to connect logical routers
together. Static routes are used to select between different logical router
ports when exiting a logical router.

Reported-by: Na Zhu <nazhu@cn.ibm.com>
Reported-by: Dustin Lundquist <dlundquist@linux.vnet.ibm.com>
Reported-at:
https://bugs.launchpad.net/networking-ovn/+bug/1545140
https://bugs.launchpad.net/networking-ovn/+bug/1539347

Signed-off-by: Steve Ruan <ruansx@cn.ibm.com>
[guru@ovn.org provided the unit test.]
Co-authored-by: Gurucharan Shetty <guru@ovn.org>
Signed-off-by: Gurucharan Shetty <guru@ovn.org>