* Created Model Classes for NVA powershell object.

* Added *-NetworkVirtualAppliance commands.

* Added *-NVASite commands and export commands.

* Added tests for NVA CRUD operations.

The tests for NVA Site CRUD operations is in skipped state.
Reason for this is a bug in the Nfv-RP which fails the
New-AzVirtualApplianceSite command. The fix is underway and
Nfv-RP team will record the tests and push it in a separate change.

* Added help files for the new 11 Cmdlets.

* Added entry in the Changelog

* Bug fix for null NVA Id.

* Added online version to help files.

* Replace real guids with 0000.. in examples.

* Add default resource paramsets, rename plural cmds

* Supress static analysis errors.

Co-authored-by: Neel Bhavsar <nebhavsa@microsoft.com>
Co-authored-by: Yabo Hu <yabhu@microsoft.com>