Add support for config action in Powershell

Same as adding additional storage accounts, metastores, we can have a list of config actions to add to a cluster before creating it, such as:

$config = Add-AzureHDInsightScriptAction -Config $config -Name "Install R" -Parameters "test" -Uri "http://r-installer.ps1" -ClusterRoleCollection "HeadNode,DataNode" | Add-AzureHDInsightScriptAction -Config $config -Name "Install Spark" -Parameters "test" -Uri "http://spark-installer.ps1" -ClusterRoleCollection "HeadNode,DataNode"

I think Dave and me already discussed this with Maxim, @maxluk , can you please confirm?