arp responder periodic timer should refresh node picked for packet-out

when gatewayMetaData in GatewayMacResolverService.java selects the node to
be used, it does not account for cases where node picked goes away.

a better approach would be to – at least in cases of failure – elect a
new node.

[16:42:09] <flaviof> vishnoianil: out of whack question on: gatewayMetaData.getExternalNetworkBridgeDpid()
[16:43:05] <flaviof> if node selected for arp responder 'goes away'... does the periodic timer select a new one?
[16:44:42] flaviof seeing error "MAC address for gateway 192.168.111.254 can not be resolved, because external bridge 38308133524292 is not connected to controller." ... and can't find 38308133524292 (0x22D74EB0D344) anywhere in md-sal: https://gist.github.com/c000aa567944ebd85acb
[16:57:40] <vishnoianil> flaviof, Yup, that's the case
[16:57:58] <vishnoianil> flaviof, and we should fix it, can you open a bug for this, i will fix it.
[16:58:15] <flaviof> no biggie. thanks
[16:58:30] <flaviof> will open a bug, just so it does not fall thru cracks

===

$ ~/bin/showOvsdbMdsal.py --ip 192.168.50.1 -f
aliasMap:
alpha -> openflow:257618520339010 br-int 00:00:ea:4d:7c:64:72:42
bravo -> openflow:8796755900804 br-ex 00:00:08:00:27:82:b9:84

ovsdbNode:192.168.50.20:47805 mgr:192.168.50.1:6640 version:2.3.2
alpha:br-int
of:1 tap79adcba5-19 mac:fa:16:3e:8f:70:a9 ifaceId:79adcba5-19e0-489c-9505-cc70f9eba2a1
of:2 patch-ext
of:3 tap341ceaca-24 mac:fa:16:3e:8e:b8:05 ifaceId:341ceaca-24bf-4017-9b08-c3180e86fd24
bravo:br-ex
of:1 patch-int
of:2 eth2

ofLinks (discover via lldp):
alpha:2 <-> bravo:1