Heketi client library

Heketi (https://github.com/heketi/heketi) is a high-level GlusterFS management service that works with an async REST API and jwt (http://jwt.io/) as the authentication mechanism.

A partial client library is required by Manila's glusterfs drivers that provides a synchronous python interface to the volume management (https://github.com/heketi/heketi/wiki/API#volumes) and partly to the node management (https://github.com/heketi/heketi/wiki/API#nodes) features of Heketi.

This library can potentially be extended to a full fledged standalone Heketi client library but that's beyond the scope of this effort. On the other hand, if a sufficiently generic Heketi library appears out of Manila we can merrily rely on that, exempting ourselves from rolling our own.