If a fixed_network and fixed_subnet is specified no private network
is created by the templates and the specified network is
used instead for VMs provisioning, like in the Ironic driver.
Currently missing is the code to handle the use case where you
specify a fixed_network but not a fixed_subnet, this will come
in a following patch.
Partially Implements: blueprint decouple-private-network
Change-Id: I2003eb709b22b905063d846eb71570fc5e033618
22fb89a6e1