PR submission to support InternalLoadBalancer, disk host caching option, capture vm image and few bug fixes

This PR contains code changes to support following work items/issues

* Support for internal load balancer [ https://github.com/MSOpenTech/azure-sdk-tools-xplat/issues/90 ]
* Disk host caching while attaching disk [  https://github.com/MSOpenTech/azure-sdk-tools-xplat/issues/78   ]
* Support for regionwise VNet [ https://github.com/MSOpenTech/azure-sdk-tools-xplat/issues/106 ]
* Support for capturing VM as VM image [ https://github.com/MSOpenTech/azure-sdk-tools-xplat/issues/109 ]
* Bug fix: unable to create affinity group as a part of vnet creation [ https://github.com/MSOpenTech/azure-sdk-tools-xplat/issues/107 ]
* Bug fix: don't re-throw 404 error from get network config [ https://github.com/MSOpenTech/azure-sdk-tools-xplat/issues/111 ]

Note : Loadbalancer set command will throw error due to hyak not updated to latest version. This issue has been tracked here https://github.com/MSOpenTech/azure-sdk-tools-xplat/issues/110 

Changelog makes very much sense. We will follow this going forward. Have changed both changelog and the help text change