[Sahara] Heat not always can configure neutron network

Even more: neutron_management_network is now required for both template and cluster. There is no need to require it in both places (actually it is not required at all).

Looked to the code, neutron_management_network is required, but it can be set in one place - template or cluster.

Andrew, I understand the point of not requiring neutron_management_network in both places (although here unfortunately we are stuck with deprecation period as these properties are part of Juno release already), but could you explain more why it is not required at all? what would happen if I create a cluster when

- it is nova-network and I pass this propety?
- it is neutron with multiple networks and I do not pass this property?

>it is nova-network and I pass this propety?
Sahara will return validation error

>it is neutron with multiple networks and I do not pass this property?
Sahara will return validation error

So, neutron_management_network must be provided in case (and only in case) of neutron. It should be provided either in cluster template or in cluster (or in both, value from cluster will be used in this case).