Without this patch, it is impossible to create a cluster without
defining a fixed_network or a fixed_subnet that already exists since we
get a Fixed{Network,Subnet}NotFound error, and Heat is unable to create
these for us.
Story: 2002652
Task: 37201
Change-Id: I0e26682b0b6093b215393eb4ce8e94eae8e5e8f7
Signed-off-by: Bharat Kunwar <brtknr@bath.edu>