Manual allocation and management of fabric links

[mikewiebe]

Placeholder issue to support manual fabric link creation

• Need to map interface from schema under fabric link section to interface under topology/switch/interface section
  • https://wwwin-github.cisco.com/netascode/nac-dc-vxlan/blob/master/schemas/fabric_schema.yaml#L66
• Creating fabric links will create the interfaces so we need to ensure we don't attempt to create the interface again in the interface create section
• Support deterministic IP address allocation
  • Fabric setting manual_underlay_ip_allocation

[juburnet]

@mikewiebe - @ccoueffe can comment and include the workflow for this. We will need this for a customer soon.