VPN instances are not cleaned up from compute node

[Oleg Bondarev]

OpenStack version: Pike
tenant networks: VxLAN

Steps to reproduce:
 - run tempest API tests for BGPVPN
 - note: several bgpvpns are not cleaned up after the tests: http://paste.openstack.org/show/724114/
 - run tempest scenario test for BGPVPN (as of Pike) - passes
 - note: 2 local_routes_count and 2 vpn_instances_count left after the test: http://paste.openstack.org/show/724113/
 - traces in ovs agent log: http://paste.openstack.org/show/724132/
 - if run scenario test again - it fails

Note: if not running API test before (hence no left bgpvpns) - all is good, not left routes, no traces in logs.