[Documentation] Node Metadata Manager design

[kimeunju108]

This PR presents the documentation of node (meta data) manager design and minor change of MAC manager design document.

• Node manager design document contains the following contents:
  • Requirements' description
  • Rest APIs and samples
  • Database Schema & System Structure, and References
  • File name: node_manager_metadata.adoc
• Mac manager document
  • Fixed anchor error
  • Added one Rest API(Create MAC State From Range)
  • Changed image file name (e.g. service_macstate.png ==> macmanager_macstate.png) for consistency with other services' documents.

[xieus]

The design doc structure is recently changed. Please merge from latest alcor/master.
(1) Node manager design is spitted into two documents:
Part 1 please goes to https://github.com/futurewei-cloud/alcor/blob/master/docs/modules/ROOT/pages/mgmt_services/node_metadata_manager.adoc
Part 2 please goes to https://github.com/futurewei-cloud/alcor/blob/master/docs/modules/ROOT/pages/infra_services/network_health_monitor.adoc

(2) Images go to https://github.com/futurewei-cloud/alcor/tree/master/docs/modules/ROOT/images

I will continue review your content change while you make the change.