Extensions for Sprint 29

@ismetkahraman @hyonholee

Could you please help review the codes for extensions? 

It also contains the following fixes:
Fix #2247 - Get-AzureService does not show DateModified correctly; 
Fix #2214 - Get-AzureVnetSite does not show Vnet IDs correctly.
Fix #2262 - Get-AzureVM | Add-AzureProvisioningConfig | Update-AzureVM generates Duplicate input endpoint exception.