addons: bridge: down: when ifreload_down_changed=1: purge bridge and upper devices...

addons: bridge: down: when ifreload_down_changed=1: purge bridge and upper devices cache

On ifreload (down ops) we need to purge the cache entry of the bridge and its upper devices
to avoid stale values in our cache.

ifup this config, then remove bridge-vids 20, ifreload: since the bridge is removed because
of ifreload_down_changed=1, we need torecreate the vlan bridge.10 and it's configuration, the
cache is stale. We need to clear it to remove the ip 10.10.10.10/32.

auto bridge
iface bridge
      bridge-ports swp1 swp2
      bridge-vids 10 20
      bridge-vlan-aware yes

auto swp1
iface swp1
      link-speed 10000

auto swp2
iface swp2

auto bridge.10
iface bridge.10
      address 10.10.10.10/32

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>