Implementation of tags for resources and resource groups

* Added support for tags on resource level (1561924)
* Added support for tags on resource group level (1561924)
* Get resources by tag (1561927)
* Added Set-AzureResourceGroup to update a resource group with tags (1561928)
I agree. I actually want to update flow in other cmdlts to have the same logic. IMHO making properties private and dynamic is better since object updates by the user will get reflected in the output.