Fix OVS agent reclaims local VLAN

If a port disappears on an OVS agent, the port is removed of the lvm
vif_ports list. And if it's the last port of the network on this agent,
the network local VLAN is reclaim.

Change-Id: I07e03107eb86a84eeb8e4d06f27a1d2fbd7cea57
Closes-Bug: #1252284
(cherry picked from commit a5d996e381bd342f0204fcd99e8548cf28b20e92)
Merge "Fix OVS agent reclaims local VLAN" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable