Implement network reservation feature

Summary
========

Support network reservation by adding new network resource-plugins.

Problem description
================

It is only "compute host" that can be reserved by Blazar for now. A variety of resources
should be supported by Blazar to improve the flexibility of resource reservation. A typical
one is "network."

Use Cases
---------------
- I want guaranteed access to 4 instances with 1 vCPU and 1GB of RAM and 10GB of disk
  and a guaranteed minimum bandwidth of 1Gbps between the instances.
  (Capacity management user story [2])

Proposed change
==============

Blazar basically respects extensibility by separating resource-plugins from other
components (e.g. manager, DB). A new "network-plugin" will be added to resource-plugins
to support network reservation.

What we have to do for the first is to define and classify network resources. Then decide
which resources to be handled by Blazar, and plan how to implement them.

Alternatives
-----------------
TBD

Data model impact
--------------------------
TBD

REST API impact
------------------------
TBD

Security impact
----------------------
TBD

Notifications impact
----------------------------
TBD

Other end user impact
--------------------------------
TBD

Performance impact
-----------------------------
None.

Other deployer impact
--------------------------------
TBD

Developer impact
---------------------------
TBD

Dependencies
===========

TBD: Related Neutron and Nova features and APIs should be investigated.

Testing
======
TBD

Documentation impact
==================
TBD

References
=========

[1] https://github.com/openstack/nova-specs/blob/master/specs/pike-template.rst
[2] https://github.com/openstack/openstack-user-stories/blob/master/user-stories/proposed/capacity-management.rst

Note: this form is based on the nova-specs template[1].