Accounts can hold multiple vnets. Same vnets can exist across multiple pnets. It is sufficient to constrain a single vnet in a given pnet within a datacenter. `i_op_dc_vnet_alloc__vnet__data_center_id` covers all use cases for the table.

Having `i_op_dc_vnet_alloc__vnet__data_center_id__account_id` will restrict
same account holding the same vnet across different physical networks.

Signed-off-by: Venkata Swamybabu Budumuru <venkataswamybabu.budumuru@citrix.com>