Share the fixed network for Ironic

When using Ironic, ensure the private Neutron network is shared across
all tenants.  This allows connectivity to the host's TFTP server from
nodes booted accross multiple tenants and more closely resembles the current
real world Ironic PXE+IPMI use cases.

Change-Id: I5d293493bafa37b1796f47aa738cfff2e3be9f59
Merge "Share the fixed network for Ironic"