Addint Test cases for bugs (TFS ids) 1464473,1472873,1427420,1290257,1431959

1464473 -> Hide OSImage and VMImage properties in Get-AzureVMImage
1472873 -> Deserializing an empty public config on Get-AzureVMCustomScriptExtension throws a value is null error
1427420 -> Stop Get-AzureVMAccessExtension displaying user password in plain text
1290257 -> Enable BGInfo by default when creating a VM with GA enabled
1431959 -> Cannot create password-less Linux VM with New-AzureVM