[DO NOT MERGE]Yun/syndication (#463)

* Update offline syndication tool to support new product kind.

* Update readme file.

* remove module, add resource group as parameter.

* Add AzsCredential option for Import-AzsOfflineMarketplaceItem.
Re-acquire access token in case token expires.

* improve log message, extend wait time for async operation.

* add tenant id match in token selection

* bug fix, pass the optional resource group parameter.

* remove assumption of registration name starts with 'AzureStack'.

* Use regular download if source uri is not in storage account. Improve
log messages.

* fix file extension bug.

* Fix Test-AzSOfflineMarketplaceItem bug.

* print correct product id message.

* resolve relative path if user provide Destination as relative path.

* simplify user interaction when choose download method

* Address review comments.

* change some log message to debug level

* avoid redownload product if it already exists.

* update readme doc

* add json file check for already downloaded product.

* adjust log message

Author: yunzhangMS