openstack backend support for libswarm

----Use case----
* New backend support is developed in libswarm for 'openstack'
* considering, openstack community is planning to come up with a nova-docker driver for container support, having a libswarm backend for openstack will make it easier to host container cloud on openstack

----Change History----
* openstack backend is added in libswarm
* backend supports 'ls', 'spawn', 'start', 'stop', 'delete' operations
* 'delete' is a new API/operation support added to libswarm

-----Dependency-----
*  https://github.com/nadgowdas/openstack 
This repo maintains a "go"lang gateway for openstack APIs. It essentially contains "go"land implementation for various openstack APIs. I have maintained it separatly so that any other container orchestrator can also use it.
****NOTE***** I am ready and willing to contribute this package into libswarm if required.

* https://github.com/goinggo/mapstructure
This is a temporary helper package currently used to use 'japth xquery' to parse json.
I plan to develop my own parser and remove this dependency in future developments.

-------Docs & Test Cases------
* Docs and test cases are committed into a new directory "docs"