Update reference-openstack-manager.md

[giuseppegaleota]

In the inputs.yaml file of the OpenStack blueprint manager, I see:

• management_network_name: cloudify-management-network
• management_subnet_name: cloudify-management-network-subnet
• management_router: cloudify-management-router
• manager_security_group_name: cloudify-sg-manager
• agents_security_group_name: cloudify-sg-agents

So, I think we should add these lines.