Implement volume reservation feature

Summary
========

Support volume reservation by adding Cinder resource-plugin.

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 "volume."

Use Cases
---------------
- TBD

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

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

Alternatives
-----------------
None.

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 Cinder 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

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