NBI part for Dashboard-related VIM info

[papagap]

Commit an API spec (using any file type or tool you want) for the NBI part of the Infrastructure Abstraction that is used to deliver VIM info required by the 5GCity Dashboard (and potentially others).

Examples are "quotas", "resource utilization info", and similar.
Coordinate with the Dashboard developers to make a list and then design the NBI spec (again in OpenStack-agnostic terms, though, of course probably OpenStack-inspired).

This should later lead to extensions of the OpenStack client (currently at https://github.com/5GCity/5GCity-slice-manager/tree/master/src/clients and later in the Infrastructure Abstraction component).

[sechkova]

Initial steps:

• Initiate a discussion with Dashboard developers to collect a list of requirements
• Verify that they match the existing OpenStack NBI and capabilities
• Merge this list with issue NBI part for VIM core entities management #1