Customdata

Add support for the CustomData field to allow passing of data to Linux VMs at provisioning time.  

Includes a test cases to validate this as well.  
```Add-AzureProvisioningConfig``` and ```New-AzureVM``` should also be updated accordingly? According to our design in terms of functionality, ```New-AzureQuickVM``` is supposed to be a subset of the ```New-AzureVM``` related ones, which means any feature exposed in the quick one, should also be included in the other one; but not vice versa.