Make subnetroute label and flow management more robust

Make subnetroute label and flow management more robust

This fix is intended to address two major problems:
a. Increasing number of stale flows in table 20 for subnetroute specific
labels.
b. Address stale flows in table 20 that always appears during bulk
migration (or) bulk evacuation.

Due to various events like Port UP/DOWN , VM migration etc , NH-DPN
election is triggered for a given subnet. This leads to update in the
SubnetRoute FIB entry. Suspicion is that it leads to un-necessary flow
creation. Since SubnetRoute flows are programmed on all DPNs whereever
there is a VPN presnce, its decided not to allocated label everytime there
is some event(like Port UP/DOWN) and update the FIB entry. To minimize
stale flow creation, MPLS label will be created only once during
subnet-addition and deleted when subnet is deleted.

Signed-off-by: Tanmay Agrawal <tanmay.a@altencalsoftlabs.com>
Change-Id: I5f1c057554eb2a1e331d87fe3f8feca99f67e57c