Dev

New cmdlets:

Get-AzureVMAvailableExtension
 Get-AzureVMBGInfoExtension
 Set-AzureVMBGInfoExtension
 Remove-AzureVMBGInfoExtension

Modified cmdlet:

Add-AzureProvisioningConfig now enables GA in default and has -DisableGuestAgent parameter for disabling GA.

...And some bug fixes.