When spawning VMs attached to pre-existing networks murano used to
generate a Heat template with a fixed_ips property for Neutron ports.
This can cause a policy violation if the target network is not owned
by the deploying tenant (i.e. the network is shared by some other
project).
This has been addressed: ExistingNeutronNetwork class no longer
generates the fixed_ips property of the port if the target network is
not owned by a current project.
Change-Id: I0c60a522f4223fdc47f87b950da1a0822a8cbdbe
Closes-bug: #1644797
247c4bdfb9