ml2: gre, vxlan type driver can leak segment_id

When multiple region of segment_id is specified, segment_id can leak
(will be never reused) on deletion of network.

Change-Id: Ia27b7e2c0ff4ab58be304e54123a679a5e35ab00
Closes-Bug: #1260262
Merge "ml2: gre, vxlan type driver can leak segment_id" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable