fix l3-agent restart with last runtime fip for dvr

fix l3-agent restart with last runtime fip for dvr

In DVR enabled environment, after we associated a floating
IP to a VM, when we restart L3-agent on the same compute
node, the L3-agent will miss to create rtr_fip_subnet for
router_info. The previous floating IP can still work, but
new associated floating IPs to VMs related to the same router
on this L3-agent will fail to configure and not work. This
patch will fix this.

The method create_dvr_fip_interfaces in dvr_router.py will
invoke fip_ns.create_rtr_2_fip_link, and the later one will
create rtr_fip_subnet, consider VMs related to the same router
will share the same rtr_fip_subnet, so processing here should
run only once for those VMs, once rtr_fip_subnet is created.
Current code will check dist_fip_count then decide to invoke
fip_ns.create_rtr_2_fip_link or not.

dist_fip_count should be zero if a router related VMs never
have been associated with any floating IPs before. But if a
router has floating IPs associated to its related VMs, after
it is restared, dist_fip_count will be non-zero, and this is
the point this patch try to fix. And for case rtr_fip_subnet
has been created, both dist_fip_count and is_fisrt will be
false, and fip_ns.create_rtr_2_fip_link will be no more need
to be invoked.

Change-Id: I3786eab86755a403991728ccb72d03f159ff8b63
Closes-Bug: 1434824