CLOUDSTACK-6072: Release VNIs when guest networks are destroyed

submitted-by: mail@ynojima.net
CLOUDSTACK-6079:
fixed issue: if public traffic is on a physical network isolated by VLAN,
and guest traffic is on an another physical network isolated by VXLAN,
public traffic's broadcast type is updated to VXLAN by mistake.

submitted-by: mail@ynojima.net