flat network type is not mandatory. There are two cases,

1. ironic with flat network: we can use neutron vlan network to
   provision baremetal nodes, as long as the interface of the baremetal
   nodes are configured to use the same vlan.

2. ironic with neutron network: all provision and cleaning_network can
   be vlan type and no flat is needed at all.

So we should remove the task.

Change-Id: I176ded6d2a8b14e350f665f63bc37eb488d32679
Closes-Bug: #1725170